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s 

551.49 

N7MVAS 

1988 



S 5 . 



VOLUME 1 



MISSOULA VALLEY AQUIFER STUDY: 
HYDROGEOLOGY OF THE EASTERN PORTION OF THE MISSOULA AQUIFER, 

MISSOULA COUNTY, MONTANA 



Prepared for 

Water Development Bureau 
Montana Department of Natural Resources and Conservation 

Helena. Montana 



by 

William W. Woessner 

Associate Professor 

Department of Geology 

University of Montana 

Missoula, Montana 59812 



Sr/ITF nnrnMrNTS COLLECTION 
OCT 2 2005 

MONTANA STAIE LIbKARY 

1515 E. 6th AVE. 
.yELENA. MONTANA 59620 




wcrcR necE 
LINE OF EQUAL ELEVATION (FT.) 



Figure S.2: Water table aap for June 1986. Arrow* indicate general direction of ground water flow. 



December 16, 1988 



Montana State Librarv 

, ii*^^^^^^^^ 

3 0864 1003 8325 9 



6002 6 z aio 



MISSOULA VALLEY AQUIFER STUDY: 
HYDROGEOLOGY OF THE EASTERN PORTION OF THE MISSOULA AQUIFKR, 

MISSOULA COUNTY, MONTANA 



Prepared for 

Water Development Bureau 
Montana Department of Natural Resources and Conservation 

Helena, Montana 



by 

William W. Woessner 

Associate Professor 

Department of Geology 

University of Montana 

Missoula, Montana 59812 



December 16, 1988 



Digitized by the Internet Archive 
in 2013 



http://archive.org/details/missoulavalleyaq1988woes 



EXECUTIVE SUMMARY 



INTRODUCnOH 



The people of the Missoula Valley have used both surface and ground water 
for their municipal water supply since the mid-1900's. Rattlesnake Creek 
supplied surface water, and wells drilled into the valley floor have provided 
ground water. In the summer of 1983 giardia contamination forced the 
abandonment of the Rattlesnake Creek water supply system and valley residents 
became dependent solely on ground water, which comes primarily from the 
Missoula Aquifer. 

The aquifer is unconfined and composed primarily of a 100 to 150 ft thick 
sequence of sand, gravel and cobbles. Below this surficial aquifer are over 
2,000 ft of fine grained sediments overlying bedrock. The absence of any 
highly productive aquifers below the Missoula Aquifer together with the coarse 
grain size of the sediments at the top of the aquifer accent the need for 
careful management of this resource. This management must sustain sufficient 
water quantity and good water quality to meet the requirements of the Missoula 
area. 

GOALS AND OBJECTIVES 

This study has two goals. The first goal is to provide a scientific 
foundation upon which short and long terra resource management decisions can be 
made. The second goal is to build on the pre-1985 data base to provide 
background for measuring existing anthropogenic effects and for assessing the 
potential for future problems. Specific objectives include: 

1 . Description of the physical properties of the Missoula Aquifer; 

2. Initiation of a long term water level monitoring system for the 

aquifer; 

3. Documentation of the seasonal variations in the water table; 

4. Delineation of the areas recharging the aquifer, the general 

direction of ground water movement within the aquifer, and the 
locations of aquifer discharge areas; 

5. Quantification of the volume of water recharging the aquifer and 

the volume removed by pumping and natural discharge; 

6. Description of the chemical properties of the ground water and 

documentation of existing and potential sources of 
contamination; and 

7. Development of a computerized numerical model of the aquifer to 

assess the impacts of natural and induced variation in recharge 
to the aquifer and in increased ground water removal by pumping. 

STUDY RESULTS 

A. Aquifer Stratigraphy 

The Missoula Aquifer is apparently composed of three llthologlc units 
which are identifiable throughout a large part of the aquifer. All contain 



sand and gravel but have other features which determine their hydrologic 
character. The top unit (Unit One) is 10 to 30 ft thick, bouldery and 
generally lies above the saturated zone of the aquifer. The middle zone (Unit 
Two) is 40 ft thick and appears to have reduced water transmitting capacity 
due to its fine grained nature. The basal unit, Unit Three, iS 50 to 100 ft 
thick, very coarse and currently yields large quantities of water to the many 
wells developed in it. Depending on how fine the grain size of Unit Two is, 
Unit Two's presence above the productive basal zone may protect underlying 
portions of the aquifer surface sources of contamination. 

B. Hydraulic Properties 

Overall, the Missoula Aquifer has a porosity of 0.20, a specific capacity 
of 0.12, a hydraulic conductivity of 18,200 gpd/ft^ and a transmissivity of 
1,152,000 gpd/ft. These values vary spatially depending on the presence and 
thickness of Unit Two and the thickness of the saturated portion of Unit One. 
As an example, hydraulic conductivity values are as high as 25,500 gpd/ft^ for 
Units One and Three and about 8,000 gpd/ft^ for Unit Two. Hydraulic 
conductivity and transmissivity values are apparently extremely high for Unit 
Three. Large production wells pump water from this unit with little drawdown. 

C. Variations in Water Table Position 

Five significant trends in the water table position are observed. The 
first is an annual water level rise which occurs between about March and June, 
and then a general decline until the following February or March. Second, the 
elevation of the spring peak water level at all wells decreased in 1987 from 
1986. Minimum water table elevations were also lower in 1987 than 1986. 
Third the seasonal water table fluctuations decrease with increasing distance 
from the Clark Fork River or other influent streams. Fourth, wells located in 
the Missoula Aquifer, near the mouths of Grant Creek and possibly Rattlesnake 
Creek, have the largest annual fluctuations in water table elevation. 
Finally, the ground water system is also affected by flow in the Bitterroot 
River, as indicated by wells located near the Bitterroot River. The maximum 
water table elevation at these wells coincides with the spring runoff peak 
stage of the Bitterroot River. 

Analysis of the long term water level trends in the valley indicate the 
effect of climatic influences and pumping stress on the aquifer. Hydrographs 
illustrate the longer term trend towards a general lowering of the valley 
water table since 1983. This trend is observed as a decrease in the 
elevations of yearly water table maxima and minima between 1983 and 1987. 
This record is too short to adequately assess whether a long term trend of net 
annual decline is actually occurring. This apparent lowering of the water 
table can be attributed to increases in withdrawal by Mountain Water Company 
starting in 1983 and to decreases in the Clark Fork River annual and peak 
discharges which also began in 1983 and continued through 1987. 

D. Ground Water Movement and Recharge-Discharge Relationships 

The sai^e general pattern of ground water flow is observed through most of 
the year. North of the Clark Fork River, ground water moves away from the 

ii 



channel and away from the northern aquifer boundary, where ground water from 
the Tertiary sediments and the alluvium in the Grant Creek and Rattlesnake 
Creek Valleys recharges the aquifer. The net result of recharge from these 
different sources is ground water migration parallel to the river channel and 
final discharge to the Clark Fork River north of the river's confluence with 
the Bitterroot River. South of the river ground water flows southwest towards 
the Bitterroot River and towards the confluence of the Clark Fork and 
Bitterroot Rivers. 

Water recharges the Missoula Aquifer by leakage from the Clark Fork 
;River, direct precipitation on the aquifer, inflow from the adjacent Tertiary 
Sediment and Bedrock Hydrostratigraphic Units, storm water runoff, septic 
system percolation and leakage from irrigation ditches. Leakage from the 
seven mile reach of the Clark Fork River in the valley constitutes over 90% of 
the recharge to the Missoula Aquifer. 

Water discharges from the aquifer by evapotranspiration, as base flow to 
streams and by pumping. Evapotranspiration rates are quantified and assumed 
small. Ground water is discharged by pumping of wells owned by individuals, 
industrial users, and two private water companies. Mountain Water Company and 
Clark Fork Water Company (CFWC). On the average, these wells pump greater 
than nine billion gallons per year from the aquifer. The volume of natural 
discharge from the Missoula Aquifer was approximated (assuming no net change 
in storage) by subtracting estimated annual recharge to the aquifer from 
annual withdrawal from wells. Estimated natural discharge is 93% of the 
annual recharge. 

Aquifer recharge exceeds the estimated ground water withdrawal by 15 
times. The leakage of water through the bed of the Clark Fork River accounts 
for over 90% of annual recharge. Therefore, observed water table declines 
most likely reflect recent climatic changes which have caused reductions in 
recharge rates from the river and Tertiary sediments. 

E. Water Quality 

The ground water in the Missoula Aquifer is calcium bicarbonate water. 
Near the Clark Fork River, mineral content of ground water fluctuates 
seasonally in response to changes in water quality of the river. Throughout 
the rest of the aquifer, mineral content remains essentially constant at any 
one point but increases in the direction of ground water flow. Water is ,: 
generally good quality and meets drinking water standards. 

Water quality steadily decreases in the down gradient direction from the 
Hellgate Canyon area east and southeast toward the Bitterroot River. For 
example, total dissolved solids increase from a low of about 240 mg/1 to a 
maximum of 358 mg/1 over distances of four to five miles. This down gradient 
increase in total dissolved solids is probably caused by natural dissolution 
in the aquifer of carbonate minerals, such as calcite and dolomite. The 
increase in calcium, magnesium and bicarbonate concentrations correlates with 
the increasing levels of total dissolved. Concentrations of other major ions, 
such as sulfate, sodium, and chloride, do not increase consistently with 
increasing total dissolved solids. 

i i i 



Concentrations of chloride and nitrate also Increase west and south west 
from the Clark Fork River along paths of ground water flow. These two Ions 
are important Indicators of pollution. Though concentrations do not exceed 
drinking water standards, above background values In Isolated portions and In 
much of the western portion of the aquifer indicate ground water degradation 
occurs in these areas. The probable sources of contamination are wastes from 
seepage rings and septic tanks and from road salt In commercial areas. 
Testing of 98 wells for collform bacteria resulted In 18 positive tests, an 
indication septic system waste is entering the ground water system. In 
addition to widespread changes in water quality, a number of Isolated events, 
such as Improper herbicide disposal, have affected portions of the aquifer and 
illustrate the vulnerability of the aquifer to pollution. 

F. Ground Water Flow Modeling 

Significant progress was made in defining the distribution of aquifer 
properties and boundaries and in understanding the Interaction between the 
Clark Fork River and the ground water system. Efforts to construct and 
calibrate a transient model of ground water flow through the Missoula Aquifer 
were unsuccessful. Future modeling efforts will be used to describe the 
responses of the aquifer to variations In Clark Fork River recharge and to 
pumping by Mountain Water Company; additional data are required to develop the 
next model. 

CONCUJSIONS AND RECOMMENDATIONS 

The results of this two year study lead to the following conclusions: 

1. The Missoula aquifer is stratigraphlcally complex. It is composed 
of three units, the second of which is not always present. The upper 
most unit. Unit One, is 10 to 40 ft thick and is composed of Interbedded 
boulders, cobbles and gravel. The middle zone. Unit Two, is composed of 
up to 40 ft of tan to yellow silt with sand and gravel. Unit Three, the 
basal unit, is composed of 50 to 100 ft of Interbedded gravel, sand and 
silt. 

2. The hydrologic properties of the aquifer reflect site specific 
stratigraphy and depositional environments. Values of hydraulic 
conductivity and transmissivity appear to decrease southwest of the 
Hellgate Canyon and Grant Creek area. Approximate hydraulic properties 
of the entire aquifer, assuming the aquifer is acting as one homogeneous 
unit, are a porosity of 0.20, specific yield of 0.12, hydraulic 
conductivity of 18,200 gpd/ft^ and a transmissivity of 1,152,000 gpd/ft. 

3. Changes in aquifer storage are indicated by water table variations. 
These fluctuations reflect seasonal changes in the quantity of recharge 
reaching the aquifer and in the rates of ground water withdrawal by 
pumping and by natural discharge. During 1985-1986, peak water table 
elevations throughout the valley were higher than peak elevations in 
1986-1987. The water table low recorded in late winter was lower in 
1987 than the previous year. These valley wide trends appear to have 
begun in 1983, when climatic conditions apparently changed, resulting in 

iv 



less than normal Clark Fork River spring discharge and aquifer recharge. 
Also since 1983, the main water producer in the valley, Mountain Water 
Company, more than doubled its ground water pumping rate. As a result, 
a general lowering of the water table occurred at Mountain Water Company 

wells . 

4. The Clark Fork River is a losing stream and seasonally recharges the 
aquifer over a four to six mile reach. Mass balance calculations show 
that the river accounts for 90 percent of aquifer recharge. Total 
aquifer recharge is 15 times greater than withdrawal from Mountain Water 
Company wells, Clark Fork Water Company wells and approximately 4,700 
individual wells. Based on these data, the apparent valley wide decline 
in the water table since 1983 is a result of a reduction in recharge 
caused principally by lower than normal flow in the Clark Fork River. 

5. North of the Clark Fork River, the direction of ground water flow is 
strongly influenced by recharge from the boundary foothills on the north 
and from the Clark Fork River. Flow in this part of the aquifer 
parallels the river and moves west until it passes the Reserve Street 
area, where it turns south to discharge to the Clark Fork River below 
its confluence with the Bitterroot River. In the area of Missoula south 
of the Clark Fork River, ground water flows south and southwest to 
discharge to the Bitterroot River and, seasonally, to the lower reaches 
of the Clark Fork River. 

6. The water quality of the aquifer is good and does not require 
treatment prior to use in most areas. The water is dominated by calcium 
and bicarbonate ions, is low in total dissolved solids (less than 500 
mg/1) and is similar in chemistry to the Clark Fork River. The presence 
of chloride and nitrate concentrations which are elevated above natural 
background levels suggests degradation from anthropogenic sources is 
occurring. Injection of storm water was found to increase chloride 
concentrations at the water table beneath storm drains. The disposal of 
sewage by seepage rings or drain fields appears to be degrading water 
quality in portions of the valley as both nitrate and chloride 
concentrations are elevated in areas served by septic systems. 
Fortunately, concentrations do not exceed drinking water standards. 
However, coliform bacteria and fecal coliform bacteria have been found 
in individual water supplies in numbers which exceed drinking water 
criteria. The aquifer is extremely vulnerable to contamination because 
it is unconfined and consists of generally coarse material. 

7. An attempt to numerically simulate the aquifer's complex 
stratigraphy and recharge-discharge relationships was unsuccessful. The '^ 
computer model could not be calibrated using field data and then 
independently verified with a second set of field data. Problems were 
viewed as being partially attributable to inadequate definition of * 
recharge rates from the Clark Fork River and of the distribution of 
aquifer properties. 

The conclusions listed above imply that the Missoula Aquifer is an 
aerially extensive, thin, unconfined system which is geologically complex and 



vulnerable to contamination. It is Missoula's sole source of water and an 
irreplaceable resource. Protection of the Missoula Aquifer requires 
continuing long term observation and management. The following 
recommendations are based on the conclusions outlined above: 

1. Maintain a long term water level observation network. 

2. Along with a water level monitoring network, a water quality 
monitoring network should be established. Approximately 30 wells 
located throughout the aquifer and finished in various aquifer units 
should be sampled four times a year. 

3. The development of a numerical model of the aquifer should be 
continued. Long term management of the aquifer as the sole source of 
water supply for Missoula and surrounding areas requires the ability to 
test the effects of increased pumping and changes in recharge rates 
prior to their occurrence. 

It is the goal of these recommendation to provide the citizens of Missoula 
with the facts needed to make educated management decisions regarding the 
future of their source of potable water. Halting data collection and further 
work on model development would be short sighted and could result in poor 
planning and crises in managing this critical resource. 



VI 



ACKNOWLEDGMENTS 



Though this work is authored by one person it could not have been 
completed without the contributions of many individuals and groups. David 
Nimick provided final editorial work and his effort is greatly appreciated. 
Researchers Bill Clark, Bill Morgan, Mike Pottinger, Karen Wogsland, Bill 
Peery and Ross Miller contributed significantly to the project. Jim Bigley 
assisted with instrumentation and Bill Thompson, Linda Angeloni and Cas Smith 
helped with data processing. Preliminary research results were also discussed 
with the staff of the Missoula City County Health Department and local 
hydrogeologists and hydrologists. 

This project received invaluable support from the Missoula City County 
Health Department, Mountain Water Company, the Missoula County Surveyors 
Office and hundreds of residents of Missoula who allowed access to their 
property and wells. 

The citizens of Montana funded about one half of this effort through the 
Water Development Grant Program of the Montana Department of Natural Resources 
and Conservation. The Missoula County Commissioners, through the Missoula 
City County Health Department, also assisted significantly with financial 
support. Additional research funds from the US Environmental Protection 
Agency, the Montana Water Resources Research Center and the University of 
Montana's Excellence Fund supported portions of this effort. 

1 sincerely appreciate all support. 



vn 



TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY i 

ACKNOWLEDGMENTS vii 

LIST OF FIGURES , x 

LIST OF TABLES xlv 

LIST OF PLATES xv 

LIST OF APPENDICES xvi 

CHAPTER I : INTRODUCTION 1 

CHAPTER 2: STUDY AREA SETTING 3 

Physiography and Geomorphology 3 

Climate , 3 

Surface Water Hydrology 3 

Geology 7 

Ground Water Hydrology 11 

Water Quality 14 

CHAPTER 3: GEOMETRY, STRATIGRAPHY AND HYDROLOGIC PROPERTIES OF 

THE MISSOULA AQUIFER 16 

Introduction 16 

Aquifer Geometry 16 

Aquifer Stratigraphy 17 

Hydrologic Properties 27 

Results of Aquifer Parameter Determination 28 

Summary of Hydrologic Properties 30 

CHAPTER 4 : WATER TABLE FLUCTUATIONS 34 

Monitoring Well Network 34 

Results of Water Level Monitoring 37 

Historical water Level Trends 51 

CHAPTER 5: GROUND WATER FLOW: SOURCES OF RECHARGE AND DISCHARGE 62 

Ground Water Flow System 62 

Aquifer Recharge and Discharge 68 

CHAPTER 6 : AQUIFER WATER QUALITY 73 

Methods 73 

Results: General Description of Ground Water Quality 73 

Chemical Trends: Results of Spring 1987 Sampling 79 

Examples of Ground Water Contamination 85 



vm 



CHAPTER 7 : GROUND WATER FT.OW MODELING lOA 

Introduction 104 

Model Design 104 

Transient Flow Simulation • Ill 

Model Results 114 

CHAPTER 8 : CONCLUSIONS AND RECOMMENDATIONS 121 

REFERENCES 125 



IX 



LIST OF FIGURES 

Figure Page 

2.1 Location of the study area and the eastern two-thirds of 

the Missoula-Ninemile Valley (Huson to Missoula) A 

2.2 Yearly precipitation totals, 1956 to 1968 5 

2.3 Location of the USGS surface water gaging stations 6 

2 .4 Average and monthly flow for the Clark Fork River at Bandman 

Bridge above Missoula 8 

2.5 Average and monthly flow for the Clark Fork River at the USGS 

gage below Missoula 9 

2.6 Geologic map of the study area 10 

2.7 Schematic north-south cross-section of the Missoula Valley 12 

3.1 Well logs and interpreted contact between Quaternary and 

Tertiary sediments 18 

3.2 Interpreted depth to the base of the Missoula Aquifer 19 

3.3 Location of geologic cross sections , 20 

3.4 Cross section A-6 21 

3.5 Cross section C-D 22 

3.6 Cross section E-F 23 

3.7 Cross section G-6 24 

3.8 Distribution of hydraulic conductivity calculated from 

specific capacity data 31 

3.9 Distribution of transmissivity calculated from specific 

capacity data 32 

4.1 Location of project monitoring network wells and Mountain 

Water Company wells 35 

4.2 Location of monitoring wells constructed for this project 36 

4.3 Location of wells equipped with water level recorders 38 

4.4 Hydrographs of data from wells MV34 and MV35 39 

4.5 Hydrograph of data from well MV6 40 

X 



Figure Page 

4.6 Hydrographs of data from wells MV31 and MV26 41 

4.7 Hydrograph of data from well MVIO 42 

4.8 Hydrographs of data from wells MV39 and MV20 43 

4.9 Hydrographs of data from wells MV2 and MV36 44 

4.10 Hydrograph of data from well P31 45 

4.11 Hydrographs of transducer data from wells MV34 and MV35 46 

4.12 Hydrographs of transducer data from wells MV37 and MV31 47 

4.13 Hydrographs of transducer data from wells MV40 and MV39 48 

4.14 Hydrographs of transducer data from wells MV38 and MV36 49 

4.15 Hydrographs of data from well MV34 and the Clark Fork River 

at the University Walking Bridge 52 

4.16 Hydrographs of data from well MV37 and the Clark Fork River 

at the Reserve Street Bridge 53 

4.17 Hydrographs of data from the Clark Fork River and wells 

located progressively further from the river 54 

4.18 Hydrographs of data from well MV39 and the Bitterroot River 

at Buckhouse Bridge 55 

4.19 Total monthly ground water withdrawals by Mountain Water 

Company 56 

4.20 Hydrograph of data from well MV31 (MWC7) 58 

4.21 Hydrographs of data from wells MWC34 and MWC30 59 

4.22 Hydrographs of data from wells MWC19 and MWC20 60 

4.23 Hydrographs of data from well MV31 (MWC7) and the Clark Fork 

River showing the departure from the average monthly discharge... 61 

.5.1. Water table map for March 1986 63 

5.2 Water table map for June 1986 64 

5.3 Water table map for October 1986 65 

5.4 Clark Fork River channel profile and the corresponding 

elevation of the water table 66 

xi 



Figure Page 

5.5 Changes in the location of the 3,135 ft equipotential line 

during the water table decline from June 1986 to March 1987 67 

5.6 Percentage of discharge, measured at the Walking Bridge, 
recharging the aquifer over a three mile reach of the river 69 

5.7 Calculated average monthly loss from the Clark Fork River 70 

6.1 Project water quality monitoring network 74 

6.2 Location of water quality sampling sites for the spring 1987 
monitoring 75 

6.*^ Distribution of major cations and anions, November and 

December 1986 77 

6 .A Hydrographs showing seasonal variation in TDS for the Clark 

Fork River and adjacent wells and seasonal change in TDS along 

a ground water flow path from CFNWB to MV20 78 

6.5 Distribution of total dissolved solids, spring 1987 80 

6.6 Distribution of chloride, spring 1987 82 

6.7 Distribution of nitrate, spring 1987 83 

6.8 Location of positive coliform bacteria tests, spring 1987 84 

6.9 Location map for the herbicide study at the Missoula County 

Weed Control Facility 86 

6.10 Schematic diagram of sump at Missoula County Weed Control 
Facility and results of water quality and soil chemical 

analyses 87 

6.11 General area in which measurable concentrations of picloram 

and bromacil were detected in the ground water. 88 

6.12 Modeled distribution of herbicides in 1974 90 

6.13 Modeled distribution of herbicides in 1989 91 

6.14 Schematic diagram of a Missoula storm drain and hydrogeologic 
instrumentation 92 



xn 



Figure Page 

6.15 A comparison of water quality data from runoff, vadose water 
and ground water at a residential and a commercial storm drain 

site 93 

6.16 Chloride concentration and water table fluctuations versus 

time for a commercial site and residential site 94 

6.17 Iron concentration and water table fluctuations versus time 

at a commercial site and a residential 95 

6.18 Nitrate concentration and water table fluctuations versus 

time at a commercial site and a residential site 96 

6.19 Location and instrumentation of septic system study sites 98 

6.20 Schematic diagram of hydrologic instrumentation at each site 99 

6.21 Concentration of chloride in the aquifer 100 

6.22 Concentration of nitrate in the aquifer 101 

6.23 Location of ground water samples contaminated with fecal 

coliform bacteria 102 

7.1 Grid for finite difference model 105 

7.2 Model grid and location of Mountain Water Company wells... 106 

7.3 Model boundary conditions and locations of wells used to 

evaluate calibration 107 

7.4 Distribution of hydraulic conductivity used in the model 109 

7.5 Elevations of the base of the aquifer used in the model 110 

7.6 Hydrographs showing average monthly stage used in the model for 

the Clark Fork River and the Bitterroot River 112 

7.7 Modeled and measured water table elevations, October 1985 115 

7^8 Modeled and measured water table elevations March 1986 116 

7.9 Modeled and measured water table elevations, June 1986 117 

7.10 Hydrograplis showing modeled and measured heads at wells MV3 

and MV31 , 118 

7.11 Hydrographs showing modeled and measured heads at well MV22 119 



xni 



LIST OF TABLES 

Table Page 

2.1 Hydrostratigraphlc units of the Missoula Valley..... 13 

3.1 Stratigraphic zones of the Missoula Aquifer 25 

3.2 Grain size data from sieve analyses for Unit One of the 

Missoula Aquifer 26 

3.3 Estimates of aquifer properties 28 

4.1 Net water table rises from winter to spring peak 50 

5.1 Volume estimates of recharge to the Missoula Aquifer 72 

5.2 Water use from the Missoula Aquifer 72 

6.1 Water analyses of representative Missoula Valley ground 

water samples collected February 5, 1986 76 

6.2 Average water quality for all water analyses 76 

6.3 Results of EPA priority pollutant scans 85 

6 .A Septic system treatment as a mean percent removal of effluent 

constituents 97 

7.1 Schedule of injection wells used to simulate Rattlesnake 

Creek and Grant Creek 108 

7.2 Pumping schedules for Mountain Water Company municipal wells..... 113 

7.3 Leakage values used in the model 114 



XIV 



LIST OF PLATES 

Plate 

1 Air photo map showing well locations In pocket 



XV 



LIST OF APPENDICES 

VOLUME 2 

Appendix Page 

3A Well Inventory 128 

3B Computer Program PTRAN 161 

3G. Transmissivity , Hydraulic Conductivity and Specific Capacity 166 

4A Well Logs of Project Installed Monitoring Wells 169 

4B Water Level Measurements 179 

4C Mountain Water Company Monthly Total Production 236 

5A Clark Fork River Leakage Calculations , 243 

6A Water Sampling Results 249 

6B Spring 1987 Sampling Results 260 

6C Results of Organic Analyses 273 



XVI 



CHAPTER 1 
mTRODUCnON 



The people of the Missoula Valley have used both surface and ground water 
for their municipal water supply since the raid-1900's. Rattlesnake Creek 
supplied surface water and wells drilled into the valley floor have provided 
ground water. In the summer of 1983 giardia contamination forced the 
abandonment of the Rattlesnake Creek water supply system and valley residents 
became dependent solely on ground water, which comes primarily from the 
Missoula Aquifer. 

The Missoula Aquifer consists of 50 to 150 feet (ft) of coarse sand and 
gravel which directly underlie the valley bottom. Currently the aquifer 
supplies over 9.7 billion gallons of water annually. The valley is surrounded 
by hills and mountains where bedrock and fine grained Tertiary aquifers 
produce only limited quantities of ground water. Therefore the Missoula 
Aquifer is considered the valley's primary source of water. In June 1988 the 
aquifer met EPA sole source aquifer criteria. Currently, water from the 
Missoula Aquifer requires no treatment and the supply of water appears 
adequate. However the ability of the ground water system to sustain continued 
development and resist wide spread contamination is unclear. 

This study has two goals. The first is to provide a scientific 
foundation upon which short and long term resource management decisions can be 
made. The second is to build on the pre-1985 data base such that existing 
impacts can be measured and the potential for future problems can be assessed. 
Specific objectives include: 

1 . Description of the physical properties of the Missoula Aquifer; 

2. Initiation of a long terra water level monitoring system for the 

aquifer; 

3. Documentation of the seasonal variations in the water table; 

4. Delineation of the areas recharging the aquifer, the general 

direction of ground water movement within the aquifer, and the 
locations of aquifer discharge areas; 

5. Quantification of the volume of water recharging the aquifer and 

the volume being removed by pumping and natural discharge; 

6. Description of the chemical properties of the ground water and 

documentation of existing and potential sources of 
contamination; and 

7. Development of a computerized numerical model of the aquifer 

capable of assessing the impacts of natural and induced variation 
in aquifer recharge and increased ground water pumping. 

During its two year time span, this large project was divided into a 
number of smaller studies. All have been under the direct supervision and 
direction of the author. They include graduate and undergraduate thesis 
research at the University of Montana by Meyer (1985), Clark (1986), Morgan 
(19860, Sendler (1986), Bayuk (1986), Peery (1989), Pottinger (1988), Wogsland 
(1988), and Ver Hey (1987). Ground water modeling was performed by Pottinger 



(1988) and research associate Brick (1987). These recent works rely upon the 
earlier work of Geldon (1979) and McMurtrey and others (1965). Many of these 
studies are summarized in this report. The reader is urged to consult these 
referenced reports for additional detailed information. Other portions of the 
study have not been published as separate reports. Pertinent data from these 
project activities are included in the appendices. 

This report is organized into two volumes. Volume 1 contains the main 
text, figures and tables. Volume 2 is the appendix to the report. 

The chapters in Volume 1 focus on the seven specific objectives listed in 
the beginning of this chapter. Chapter 2 briefly describes the study site. 
Chapter 3 addresses the geometry, stratigraphy and hydro logic properties of 
the aquifer. CUapter A describes the ground water monitoring program used in 
this project and reports on the observed seasonal fluctuations of the water 
table. Chapter 5 quantifies aquifer recharge, aquifer discharge and 
directions of ground water flow. Chapter 6 describes water quality in the 
aquifer and summarizes three case studies of aquifer contamination. Chapter 7 
describes the attempt at modeling the physical ground water system in an 
effort to evaluate future effects on the aquifer. Chapter 8 presents 
conclusions and recommendations. 

Volume 2 contains supporting material for chapters three, four, five and six. 



CHAPTER 2 
STUDY AREA SETTING 



Numerous authors have prepared general site descriptions of the Missoula 
Valley. The one presented here is taken primarily from the work by Clark 
(1986). ,- , 

PHYSIOGRAPHY AND GEOMORPHOLOGY 

The Missoula Valley is approximately 35 ml^ in size. It is bounded by,' 
the Rattlesnake Hills on the north, the Sapphire Mountains on the east, the - 
Bitterroot Mountains on the south and, on the west, by a low plateau in the 
vicinity of the Johnson-Bell airport (Plate 1). The Missoula Valley is the 
eastern part of a long intermontane depression trending WNW from the city of 
Missoula (Figure 2.1). 

The topography of the valley floor is relatively flat, but slopes 
gradually downward from the bounding highlands towards the northwest, where 
the Clark Fork River leaves the valley. There are two major river terraces on 
the valley floor. Twenty foot scarps separate the terraces from each other 
and from the floodplains of the Clark Fork and Bitterroot Rivers (Geldon, 
1979). Another valley floor feature is McCally Butte, a 200 ft high bedrock 
outlier located in the floodplain of the Bitterroot River. 

The Missoula Valley is drained by the Clark Fork and Bitterroot Rivers. 
The Clark Fork River enters the valley from the east through the 1,500 ft deep 
Hellgate Canyon (Plate 1). The river flows westward for about seven miles, 
meeting the Bitterroot River at Kelly Island. The Clark Fork River has a 
gradient of 10. A ft/mi. The Bitterroot River enters the valley in the south 
central portion of the study area. It flows northwest for 4.5 mi at a 
gradient of 5.2 ft/mi before joining the Clark Fork River. Several smaller 
streams enter Che valley from the surrounding highlands, including Rattlesnake 
Creek, Grant Creek and Pattee Creek (Plate 1). 

CLIMATE 

The climate of the Missoula Valley is semiarid. Winter is dominated by 
Pacific maritime air, which occasionally is displaced by cold continental air 
draining through the Clark Fork Valley. Annual precipitation averages 13.29 
inches (NCAA, 1987) (Figure 2.2). Peak precipitation occurs in May and June. 
The least precipitation occurs in February and March. High intensity 
convective storms in July and August may also contribute significant 
precipitation. 

SURFACE WATER HYDROLOGY 

The Clark Fork River is gaged above and below Missoula by the U.S. 
Geological Survey (USGS) (Figure 2.3). The gaging station above Missoula is 
located near Bandman Bridge, 2.8 mi east of Missoula. The station below 




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Missoula is located I .0 mi dovmstream of the confluence with the Bitterroot 
River, 4.5 mi west of Missoula. Both have been gaged continuously since 1929. 

The drainage area of the Clark Fork River above the upper gaging station 
is 5,992 mi^ . Based on 57 years of data (1930 to 1986), mean annual discharge 
is 3,051 cubic feet per second (cfs) (USGS, 1987) (Figure 2.4). The maximum 
recorded daily flow was 32,300 cfs on June 21, 1975. The minimum flow of 340 
cfs occurred on September 27, 1937. Average monthly discharges indicate the 
general response of the river to spring runoff. The lowest mean monthly flow 
(1,596 cfs) occurs in December while spring rains and snowmelt cause the 
maximum mean monthly flow (8,740 cfs) in June (Figure 2.4). Peak discharge in 
1985 was 8,680 cfs on May 25 and minimum discharge was 826 cfs on August 1. 
In 1986, the river peaked at 12,900 cfs on June 1 and had its minimum 
discharge of 979 cfs on August 20 and 21. 

The gaging station below Missoula records the combined flow of the Clark 
Fork and Bitterroot Rivers. Mean annual discharge is 5,547 cfs (USGS, 1984) 
(Figure 2.5). A maximum discharge of 52,800 cfs occurred on May 23, 1948; the 
minimum was 388 cfs on January 18, 1933. Mean monthly discharge varies from 
2,045 cfs in December to 20,160 cfs in June (Figure 2.5). A 1985 peak 
discharge of 19,900 cfs occurred on May 25; the minimum discharge was 1,180 
cfs on August 1. In 1986 a peak of 32,100 cfs was observed on May 31 and a 
minimum flow of 1,620 cfs on August 21. The Bitterroot River contributes 
about 3,000 mi^ to the 9,003 mi^ drainage area above this gage. 

The Bitterroot River was gaged independently from 1898 to 1905 at a 
bridge four miles southwest of Missoula. For this period of record, mean 
annual discharge was 3,260 cfs (USGS, 1975). Geldon (1979) estimates the mean 
annual discharge for the Bitterroot River as 2,339 cfs, or about 40% of the 
total flow of the Clark Fork River below the confluence of the rivers. 

Rattlesnake Creek was gaged from 1959 to 1967 at the Vine Street Bridge 
in Missoula. The drainage area is 79.7 rai^ . Mean annual discharge for the 
period was 110 cfs (USGS, 1975). Geldon (1979) estimated the mean annual 
discharge for the period from 1959 to 1977 as 135 cfs by extrapolating the 
Rattlesnake Creek gaging data with Clark Fork River gaging records for the 
entire period. , 

Pattee Creek and Grant Creek have not been gaged by the USGS. Geldon 
(1979) estimated discharge for Pattee Creek by comparing drainage area and 
precipitation data for these ungaged drainages with Rattlesnake Creek data. 
Mean annual discharge is 13 cfs for Pattee Creek, du Breuil (1983) gaged 
Grant Creek and estimated the mean annual discharge at 30 cfs, 

GEOLOGY 

The sediments of the Missoula Valley are continental clastic deposits 
(Figure 2.6). The floor of the valley is covered by alluvial and lacustrine 
sediments of Quaternary age, 1.6 million years to the present. These deposits 
are described in more detail in Chapter 3, 



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Annual (top) and average monthly (bottom) flow for the Clark 
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The low foothills surrounding the valley floor are principally composed 
of fine grained sediments deposited when the basin was internally drained 
during the Tertiary period, 43 to 5.3 million years. Although up to 2,500 ft 
of Tertiary sediments are preserved in the valley (McMurtrey and others, 
1965), surface exposures are infrequent and poor around the valley margins. 
The sediments range in size from clay to coarse gravel and unconf ormably 
overlie Precambrian Belt Supergroup metasediments. Tertiary sediments are 
subdivided into three stratigraphic units: 

1. Fanglomerates (pre-Renova Formation equivalent) which are believed 

to be limited to the basin's margins, ? to 43 million years; 

2. Fine grained ash-rich sediments (Renova Formation equivalent) which 

underlie the valley floor, 43 to 20 million years; and 

3. Coarse clastic sediments (Sixmile Creek Formation equivalent) which 

are found locally in the foothills and may be overlying the 
Renova Formation beneath the valley floor, 20 to 5.3 million 
years . 

A north-south diagrammatic section of the valley showing Tertiary and 
Quaternary deposits is presented in Figure 2.7. The Cenozoic geology of the 
Missoula area has been described in detail by McMurtrey and others (1965), 
Kuenzi and Fields (1971), Fields (1981), Thompson and others (1982), 
Wehrenberg (1983) and Fields and others (1985). 

Mountain ranges surrounding the valley, including Mount Jurabo and Mount 
Sentinel, are composed of Precambrian metasediments of the Belt Supergroup, 
0.8 to 1.6 billion years. 

The interraontane depression, of which the Missoula Valley is part, is 
believed to have formed as a result of horizontal extension after Laramide 
thrusting, which occurred between late Cretaceous and middle Eocene time, 97.5 
to 52 million years ago (Fields and others, 1985). The horizontal extension 
resulted in normal faulting parallel to the faces of Mount Jumbo and Mount 
Sentinel and in the formation of the Clark Fork Fault, which is exposed on the 
north side of the valley (Figure 2.6). 

GROUND WATER HYDROLOGY 

Missoula Valley residents use ground water from three sources: fractured 
Precambrian Belt Supergroup rocks. Tertiary Renova equivalent sediments, and 
the coarse alluvium which is exposed at the surface of the valley floor (Table 
2.1). Use of the fractured bedrock and the Renova equivalent aquifers is 
generally restricted to the valley margins. The shallow sand and gravel 
deposits below the valley floor are the principal source of ground water. 

The Precambrian bedrock surrounding the valley is generally a poor 
aquifer. The rock is essentially impermeable and water flows only through 
fractures. Wells generally yield about one gallon per minute (gpm) . However, 
in the Hayes Creek area, a well yielded 17 gpm during a four hour aquifer test 
(Bayuk, 1986). Wells finished in bedrock range in depth from 38 to over 1,000 
ft. Geldon (1979) reported an average specific capacity of 0.11 gallons per 
minute per foot of drawdown (gpm/ft). Analysis of driller's reports in the 

11 



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Hayes Creek area indicates specific capacities vary from 0.5 to 35 gpm/ft 
Bayuk (1986) noted that yield decreases with increasing well depth. 



Table 2.1 
Hydrostratigraphic Units of the Missoula Valley 



Hydros t rat igraphic 

Unit 

Missoula Aquifer 



Tertiary Sediments 



Precarabrian 



Age 



Miocene (?) 
to Recent 



Late Eocene 
to Early 
Miocene (?) 



Precambrian 



Thickness 
(ft) 

150 



2,500 to 
3,500 



greater 

than 

10,000 



Description 



Sand, gravel and boulders 
with some silt and clay. 
Clasts well rounded. 
Yields up to 7,000 gpm. 

Clay with interbedded and 
embedded sand and gravel. 
Clay is blue, gray, brown, 
tan and red. Local coal 
and volcanic ash. Yields 
up to 40 gpm but averages 
25 gpm. 

Quartzite, red and green 
argillite and carbonates. 
Water is from fractures. 
Yields up to 17 gpm, but 
averages 1 gpm. 



Renova equivalent sediments which occur on the valley flanks and beneath 
the Missoula Aquifer locally comprise a second low yielding aquifer. 
Discontinuous lenses of sand and gravel, usually less than 10 ft thick, 
provide water under artesian conditions to wells, generally at less than 20 
gpm (Geldon, 1979). Barclay (1986) studied the hydrogeology of the Ninemile 
Valley and reported the average yield of 32 wells finished in Renova sediments 
to be 11.3 gpm with a range of 0.5 to 45 gpm. Finstick (1986) found similar 
values for Renova equivalent sediments in the Bitterroot Valley near Victor. 
However, well MWC3 on South Avenue (Plate 1) penetrates 2 three foot zones of 
sand and gravel in the Renova equivalent sediments which yielded 75 gpm when 
the well was developed. Geldon (1979) reported hydraulic conductivities (K) 
averaging 165 gallons per day per foot squared (gpd/ft^), specific capacities 
of 0.51 gpm/ft and a storage coefficient of 10"^. Barclay (1986) reported an 
average hydraulic conductivity of 300 gpd/ft^ with a range of 0.08 to 1,900 
gpd/ft^ for Renova equivalent sediments in the Ninemile Valley. 

Both the Precambrian and Tertiary units yield small quantities of water 
to wells. They are utilized for domestic supplies where other more productive 
sources are usually not available. In some small tributary valleys, saturated 
alluvium yields small quantities of water (Barclay, 1986). The saturated 
coarse sand and gravel material which directly underlies the valley floor is 



13 



of principle interest to the majority of water users in the Missoula basin. 
These coarse sediments include Pleistocene and Recent alluvial sediments and 
may also include an upper portion of the Sixmile Creek Formation. Although 
these sands and gravels may range in age from Miocene to Recent and be part of 
different geologic formations, they have similar hydrologic properties. 
Therefore, these sands and gravels are grouped into one hydrostratigraphic 
unit called the Missoula Aquifer. This aquifer yields over 9.7 billion 
gallons of water annually to wells which supply the city and over 4,700 
individual dwellings. Geldon (1979) reported that the Missoula Aquifer has an 
average hydraulic conductivity of 5,100 gpd/ft^, specific capacities of over 
3,000 gpm/ft and transmissivities (T) of over 1,000,000 gpd/ft. Aquifer 
storage coefficients range from 0.11 to 0.35. It is the Missoula Aquifer that 
is the focus of this report. 

Geldon (1979) prepared maps of maximum and minimum water table positions 
in the Missoula Aquifer. He noted seasonal fluctuations in the water table 
with highs in the first half of July and lows around March 1. Both McMurtrey 
and others (1965) and Geldon (1979) recognized the importance of the Clark 
Fork River in recharging the surficial sand and gravel system. They 
interpreted flow directions to be away from the Clark Fork River. Flow is to 
the west on the north side of the river and to the southwest on the south 
side. Geldon (1979) estimated the average discharge of water through the main 
part of the Missoula Aquifer to be 5.1 x 10^ ft^/d at a velocity of 6.2 ft/d. 
He attempted a water balance for the aquifer and concluded that discharge from 
the aquifer was exceeding recharge during 1977 and 1978 by about 21,000 
acrefeet (acft). 

WATER QUALITY 

Water quality in the Missoula Aquifer is good and well below all drinking 
water standards. Juday and Keller (1978), the Montana State Water Quality 
Bureau (WQB), Hydrometrics (1984) and Mountain Water Company have analyzed 
water quality in the valley. 

Ground water in the Missoula Aquifer is calcium-bicarbonate type. It is 
moderately hard, as expressed by the sum of calcium and magnesium ion 
concentrations. Total dissolved solids are usually less than 350 milligrams 
per liter (mg/1). Chloride concentrations are less than 10 mg/1 and sulfate 
is less than 30 rag/1. The pH ranges from 6.8 to 8.5. Geldon (1979) reported 
an average calcium to silica ratio of 2.3, which implies a relatively rapid 
circulation of ground water in the aquifer. The ratio decreases southwestward 
away from the Clark Fork River as the ground water flows farther from the 
recharge area. Seasonally, wells near the Clark Fork River and downtown 
Missoula show 60% fluctuations in total dissolved solids in response to 
similar changes in the Clark Fork River (Hydrometrics, 1984). The WQB 
collects an annual ground water sample in the Missoula Valley. 

The Mountain Water Company monitors water quality in the valley's 
municipal wells. Fifty-four bacteriological samples are collected each month 
(about 2 samples per well per month). Every 4 years MWC tests its wells for 
inorganic chemical constituents. They also sample the Clark Fork River 
periodically. 

14 



Water found in the Tertiary sediments is a calcium-bicarbonate type. 
Total dissolved solids are generally less than 500 mg/1 (Geldon, 1979; Juday 
and Keller, 1978). Iron concentrations typically exceed the 0.30 mg/1 
drinking water standard. Based on measured calcium to silica ratios, Geldon 
(1979) concluded that ground water circulated through the Tertiary sediments 
longer than shallower water did in the Missoula Aquifer. 

Bayuk (1986) reported water quality data for the Precambrian bedrock 
aquifer underlying the Hayes Creek area. The water is calcium-bicarbonate 
type with total dissolved solids ranging from 290 to 350 mg/1. Water quality 
is characteristic of a bedrock aquifer near its highland recharge area. It is 
anticipated that deep water below the Missoula Valley in mineralogically 
similar bedrock would be considerably higher in total dissolved solids. 



15 



CHAPTER 3 

GEOMETRY, STRATIGRAPHY AHD HTDROLOGIC PROPERTIES 
OF THE MISSOULA AQUIFER 



mTRODUCnON 

One of the specific objectives of this research effort was to describe 
the physical and hydrologic properties of the Missoula Aquifer. These 
properties include transmissivity, storage, porosity and hydraulic 
conductivity as well as the variation in aquifer thickness and internal 
heterogeneity. This chapter starts by presenting data on the aerial and 
vertical extent of the aquifer material. The second portion defines the 
hydraulic properties of the aquifer. 

AQUIFER GEOMETRY 

The Quaternary deposits shown on the geologic map (Figure 2.6) indicate 
the surface extent of the Missoula Aquifer in the study area. The aquifer 
covers approximately 35 mi^ . The vertical extent of the aquifer can be 
approximated from the hundreds of driller's reports and well logs on file at 
the Missoula office of the Montana Department of Natural Resources and 
Conservation. Summaries of these well logs are presented in Appendix 3A. 
Morgan (1986) and Clark (1986) attempted to delineate the aquifer base and the 
internal stratigraphy of the aquifer from these records. Much of the 
following description is taken from Morgan's and Clark's research. 

Aquifer geometry and stratigraphy were evaluated by interpretation of 
geologic logs recorded on driller's reports and from the collection and 
description of cuttings taken from nine wells drilled during the course of 
this project. Field inspection of excavations and road cuts in which the 
unsaturated portion of the aquifer was exposed were also examined. 

Interpretation of the aquifer base proved more difficult than 
anticipated. Less than 20 wells were inferred to fully penetrate the aquifer. 
Most of these were Mountain Water Company wells, which are concentrated in the 
eastern portion of the valley. 

Two criteria were used to determine the base of the aquifer. The first 
is a downward change, seen in the driller's geologic log, from a coarse sand 
and gravel sequence to a sequence dominated by silts and clays. This 
transition presumably separates coarse grained sediments which yield water 
from the underlying fine grained Tertiary sediments which do not yield 
significant amounts of water. 

The second criterion is based on well design and yield data recorded in 
driller's reports. Total depths and perforated intervals of wells were used 
to identify portions of the aquifer with higher transmissivity. These data 
were then used in conjunction with yield and drawdown information reported by 
drillers. Domestic wells penetrating the Missoula Aquifer produce 10 to 100 



16 



gpm with only a few feet of drawdown. In contrast, wells finished in Tertiary 
sediments produce only a few gallons per minute with tens of feet of drawdown. 

Seismic refraction work conducted by Clark (1986) was unsuccessful in 
defining the base of the aquifer, which was assumed to be 150 to 200 ft below 
land surface. Apparently the acoustic impedance between the sand and gravel 
deposit and the underlying Renova equivalent sediments is too low to be 
recognized with the equipment used (Bison Signal Enhancement Seismograph Model 
1570B), and/or the interface is deeper than what the equipment can record. 

Figure 3.1 presents data from driller's reports for three wells. Two of 
the wells penetrate the aquifer in the Missoula Valley. A contrasting third 
well is from the Nlnemile area and is finished in Tertiary sediments similar 
to those found on the flanks of the Missoula Valley and beneath the Quaternary 
sand and gravel of the valley floor. . ' 

Figure 3.2 presents the interpreted depth from land surface to the base 
of the aquifer. Although data are insufficient to construct an isopach map, 
cross sections could be constructed from well data to indicate the general 
position of the aquifer base (Figures 3.3 to 3.7). Well data used for these 
cross sections are summarized in Appendix 3A. Based on the cross sections, 
the Missoula Aquifer averages between 110 and 140 ft in thickness but ranges 
from 92 to 237 ft thick. These thickness values include the unsaturated 
portion of the aquifer. 

The saturated thickness of the aquifer ranges from 50 to 120 ft in the 
most of the valley. The thinner sections occur in the central and northern 
portion of the valley. Depth to water is typically 50 to 70 ft in the central 
portion of the basin and at its northern contact with the Tertiary sediments. 
The water table is closest to the surface (10 to 30 ft) adjacent to streams 
and in the southwestern portion of the basin. 

The Missoula Aquifer is thickest (over 200 ft) at the merger of the Grant 
Creek Valley and the main Missoula Valley. In the spring when. Grant Creek is 
actively recharging the aquifer, the saturated portion of the aquifer near the 
mouth of the Grant Creek Valley is 130 to 150 ft thick with one site recorded 
as 197 ft (Pottinger, 1988). 

AQUIFER STRATIGRAPHY 

Morgan (1986) developed a composite picture of the stratigraphy of the 
Missoula Aquifer from his extensive review of driller's reports and 
construction of cross sections (Figures 3.3 to 3.7). The stratigraphy noted 
by Morgan is based on his interpretation of often sketchy driller's logs and 
has not been confirmed by a detailed drilling program. He described four 
stratigraphic units. The lowest unit correlates with the Tertiary Sediment 
Hydrostratigraphic Unit and the other three are interpreted as parts of the 
Missoula Aquifer Hydrostratigraphic Unit (Table 2.1), These upper three units 
are renumbered and described below (Table 3.1). 



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TABLE 3.1 
Stratigraphic Zones of the Missoula Aquifer 

Unit One: Interbedded boulders, cobbles and gravel with sand, 

silt and some clay. Thickness from 10 to 30 ft. Found 
at land surface. 

Unit Two: Tan to yellow, silty sandy clay with local layers of 

coarse sand and gravel. Thickness averages 40 ft in the 
center of the basin (Figure 3.5) to 130 ft in the 
northwest section of the study area (Figure 3.7). 
Absent in places. 

Unit Three: Interbedded gravel, sand, silt and clay. Unit seems to 
be coarser at the bottom. Thickness varies from 50 to 
100 ft. May include part of the Sixmile Creek Formation, 



Unit One forms the surface of much of the valley floor. It is composed 
of 10 to 30 ft of large boulders, cobbles, sand and silt and lacks the 
abundant fine sediments of Unit Two (Figures 3.4 to 3.7). The large cobbles 
and boulders are characteristic and are difficult to drill through. Unit One 
was probably deposited by large aggrading river systems fed by glacial 
meltwaters during the last stages of the Pleistocene. 

To determine the size range and uniformity of aquifer material, sieve 
analyses were conducted on four 50 lb sediment samples collected from the 
upper unsaturated portion of the aquifer (Clark, 1986) (Table 3.2). Two 
samples came from a vertical face of the excavation for the University of 
Montana Stadium at the northeast corner of the campus and two from a gravel 
bar in the Clark Fork River between the confluence of Rattlesnake Creek and 
the University Walking Bridge. All four samples are very coarse and very 
poorly sorted. Using the modified Wentworth grain-size scale, 90% of the 
sediment is at least the size of coarse sand. Mean diameters are in the 
coarse to very coarse pebble range. Values of the uniformity coefficient and 
inclusive graphic standard deviation for all 4 samples indicate that each 
sample is very poorly sorted (Folk, 1980). 

Saturation of Unit One generally occurs only beneath and adjacent to 
streams and in the southwestern corner of the aquifer. The rest of this unit 
is above the saturated zone and therefore does not yield water. The shallow 
ground water found in saturated portions of Unit One is more likely to be 
affected by contaminants infiltrating from the surface than water found in 
deeper units. 



25 



TABLE 3.2 

Grain Size Data from Sieve Analyses for 

Unit One, Missoula Aquifer 

Sample 1 Sample 2 Sample 3 Sample 4 



Location 


excavation for 


gravel 


bar 




UM 


stadium 


Clark 


Fork 


River 


Depth of 


5-15 


15-20 


0-0.5 





.5-1.0 


sample (ft) 












Effective 


0.50 


0.55 


4.0 




0.55 


diameter (mm) 












Mean diameter 


19.8 


53.5 


79.7 




60.3 


(mm) 












Uniformity 


44 


91 


30 




104 


coefficient 












Inclusive 


2.31 


2.92 


3.18 




2.74 


graphic 












standard 












deviation 













Unit Two generally consists of a yellow silty sandy clay with interbedded 
gravel and sand lenses. Well logs do not contain sufficient information to 
determine detailed stratigraphy, but drillers report a predominance of tan 
clay, sand and gravel. Drillers note the existence of clay in this unit from 
the return of cloudy tan to pink water with the drill cuttings. However, 
cuttings of sand and gravel are also returned continuously during drilling of 
some wells. Based on its lithology and the presence of the tan clay. Unit Two 
is probably related to deposition of Pleistocene Glacial Lake Missoula 
sediments which outcrop in the northwestern part of the study area (Figure 
3.4). In fact, finer portions of the drill cuttings look very similar to 
Glacial Lake Missoula sediments. During the repeated filling and draining of 
Glacial Lake Missoula, a complex sequence of alluvial fans, stream channels 
and deltas interf ingered with lake sediments on the flanks and floor of the 
valley. Over 100 ft of fine grained lake sediments overlie gravel in the 
western portion of the valley near Huson and appear to interfinger with the 
western edge of the Missoula Aquifer (Figure 3.4). Unit Two is discontinuous 
in some areas of the aquifer and may have been replaced by coarser fluvial 
deposits (Figure 3.4 and 3.7). Few wells are developed in Unit Two because 
Unit Three has a higher transmissivity . 

Unit Three is dominated by coarse grained sediments, especially at the 
base of the unit. Tan fine grained sediments are interlayered with sand and 
gravel. The ratio of fine to coarse material must be low because wells 
pumping over 3,000 gpm are developed in this unit. Unit Three probably 
represents channel lag, point bar and floodplain deposits from a large fluvial 
network which developed over time. Morphologically similar deposits are found 
in the present Clark Fork River channel and floodplain. This large fluvial 
could have developed either in the Pleistocene or late Tertiary. 



26 



Grimestad (1977) observed similar stratigraphy west of the study area at 
the Stone Container Corporation mill near Frenchtown (Figure 2.1). He 
described a 25 to 35 ft thick, sequence of sand and gravel at the surface; an 
underlying 60 to 125 ft thick sequence of mixtures and lenses of silt, sand, 
clay and gravel; and a basal 20 to 50 ft thick zone of sand and gravel resting 
on bedrock. This sequence of finer material sandwiched between coarser units 
correlates well with the three Missoula Aquifer units described previously. 
The thickness and pronounced finer nature of Grimestad's middle layer 
(compared to Unit Two) may be a result of either being farther from the 
northeast portion of the valley where high energy streams enter, or because it 
was lower in elevation and subject to longer periods of lake related 
deposition. 

In summary, the Missoula Aquifer appears to be composed of three 
lithologic units which are identifiable throughout a large part of the 
aquifer. All contain sand and gravel but have other features which determine 
their hydrologic character. The top unit is bouldery but generally above the 
saturated zone. The fine grained component of the middle zone appears to have 
reduce water transmitting capacity. When it is present above this productive 
basal zone, it may protect portions of the valley's water supply from quick 
downward movement of contaminants through Unit One. The basal unit is very 
coarse and currently yields large quantities of water to the many wells 
developed in it. 

HYDROLOGIC PROPERTIES 

Important hydrologic properties of an aquifer include porosity, specific 
yield or storage, hydraulic conductivity and transmissivity . Characterization 
of these properties in the Missoula Aquifer was accomplished by reviewing the 
literature, interpreting driller's reports and conducting grain size analyses, 
perraeameter experiments, and aquifer pumping tests. Clark (1986) describes 
the basic methods used to interpret aquifer properties and the following 
material has been largely taken from his work. Additional results of Morgan 
(1986) are included in the discussion of specific capacities. 

All hydrologic analyses done in this study were based on the major 
assumption that the aquifer is unconfined, isotropic and homogeneous. This 
assumption was made to simplify calculations and to allow the maximum 
interpretation of data. The description of the aquifer stratigraphy presented 
above indicates that this simplifying assumption does not properly represent 
the Missoula Aquifer. However, there is currently insufficient information 
available to analyze the aquifer using a more accurate model. Even 
considering the heterogeneities of the Missoula Aquifer, aquifer testing and 
interpretation is believed to result in generally conservative estimates of 
aquifer properties. 

Four 50 gallon samples of aquifer material from Unit One were evaluated 
by Clark (1986) to determine values for specific yield, porosity and hydraulic 
conductivity. The hydraulic conductivities of the samples were also estimated 
from the sieve analysis data using techniques developed by Slichter and 
Terzaghi described in Fraser (1938). Aquifer properties derived from these 

27 



methods were considered estimates applicable to Unit One and, assuming a 
similar depositional environment, Unit Three. 

Aquifer properties for Units Two and Three were determined by analysis of 
aquifer testing data. In the first method used, drawdown and pumping 
information recorded on driller's reports were analyzed to calculate 
transmissivity using a computer program (PTRAN) (Appendix 3B) . Clark (1986) 
describes the assumptions used to develop the program. Several problems were 
encountered using this technique. First, domestic wells posed a challenge as 
they are typically finished as open ended casings and do not penetrate the 
full aquifer thickness. In an attempt to use the maximum available data, an 
aquifer penetration value of one foot was used for open ended wells. For most 
domestic wells, aquifer thickness was interpreted as the length of continuous 
saturated coarse sand and gravel extending upwards from the bottom of the well 
casing. For large water company wells, the saturated thickness was assumed to 
be the thickness of Unit Three or, if Unit Two was absent, the thickness of 
Unit Three plus the saturated portion of Unit One. Second, inaccuracies in 
measurements by drillers directly affect calculated transmissivity values. 
For instance, pumping times are often rounded to the nearest hour, drawdown in 
wells pumped by airlifting is difficult to determine and variations in 
discharge may go undetected. These problems are more common for analysis of 
data from smaller diameter wells drilled by rotary rigs than with the few 
dozen larger diameter wells drilled by cable tool. 

Aquifer testing on well MWC34 was the second method used to calculate 
transmissivity for Unit Three (Plate 1). The tests also provided values of 
horizontal and vertical hydraulic conductivity and specific yield. Clark 
(1986) describes the details of the aquifer testing. 

RESULTS OF AQUIFER PARAMETER DETERMIIIATION 

Results of aquifer parameter determination from the combined laboratory 
and field tests and from analyses of driller's reports are summarized in this 
section. Results are organized by hydraulic units and summarized in Table 
3.3. 



TABLE 3.3 
Estimates of Aquifer Properties 



Property 


Unit One 


Unit Two 


Unit Three 


Porosity 


0.20 





0.20 


Specific Yield 


0.12 





0.10 


Thickness (ft) 


10-30 


40 


50-150 


Hydraulic Conductivity 


10,300 


200 


10,300- 


(gpd/ft2) 






25,500 


Vertical Hydraulic 








970-2,100 


Conductivity (gpd/ft^) 








Transmissivity (gpd/ft) 


103,000- 


8,000 


750,000- 




310,000 




1,710,000 



28 



Laboratory determinations of hydrologic properties of Unit One revealed 
average values of 0.20 for porosity, 0.12 for specific yield, 0.08 for 
specific retention, and 10,370 gpd/ft^ for hydraulic conductivity. 

The sieve analyses of Unit One samples yielded hydraulic conductivity 
values which vary over 2 orders of magnitude (Clark, 1986). The large 
differences in calculated values are caused by the four to eight fold 
variation in the mean and effective grain diameters of samples. An average 
hydraulic conductivity value for the four samples computed with both methods 
is 2,953,710 gpd/ft2. 

Grimestad (1977) reported a hydraulic conductivity value of 10,250 
gpd/ft^ and a specific yield of 0.20 to 0.35 for the surficial gravel which 
corresponds to Unit One. 

Morgan (1986) estimated hydraulic properties of Unit Two from driller's 
specific capacity data from a small number of wells finished in what he 
interpreted as the finer middle unit. Average specific capacity was 7 gpm/ft. 
Using estimating techniques described by Driscoll (1986) which convert 
specific capacity to transmissivity , Morgan calculated a representative 
transmissivity for the unit of about 10,500 gpd/ft. Based on an average 
thickness of 40 ft, the hydraulic conductivity is approximately 260 gpd/ft^. 
Grimestad (1977) reported a hydraulic conductivity of 150 gpd/ft^ for his 
middle unit. 

Unit Three hydraulic properties were obtained from analysis of drillers's 
reports and aquifer tests. Estimates by Clark (1986) of transmissivity 
generated from specific capacity data using the computer program PTRAN are 
presented in Appendix 3C. Transmissivity values computed from specific 
capacity data range from 52 to 4,149,000 gpd/ft for domestic wells and average 
365,600 gpd/ft. Values for municipal wells range from 48,000 to 9,752,000 
gpd/ft and average 1,710,000 gpd/ft. An average value of transmissivity for 
both municipal and domestic wells is 750,000 gpd/ft. Hydraulic conductivity 
values for municipal wells range from 520 to 113,400 gpd/ft^ and average 
25,500 gpd/ft^. Values for domestic wells range from 1 to 27,700 gpd/ft^ and 
average 4,100 gpd/ft^. The average value for both municipal and domestic 
wells is 10,300 gpd/ft^. 

Morgan (1986) reported specific capacity data for over 50 wells completed 
in Unit Three and found an average specific capacity value of 240 gpm/ft for 
these wells. Estimation of hydraulic conductivity and transmissivity from 
these data using Driscoll's (1986) method yields a range of hydraulic 
conductivity values between 3,600 and 7,200 and a transmissivity of 360,000 
gpd/ft (Driscoll, 1986). 

Results of the two aquifer tests conducted by Clark (1986) provide 
transmissivity values for Unit Three which vary considerably and range from 
606,000 to 5,856,000 gpd/ft (Table 3.3). Specific yield values ranged from 
0.03 to 0.07. Depending on the assumptions used, horizontal hydraulic 
conductivity ranged from 6,400 gpd/ft^ to 62,300 gpd/ft^. Values of vertical 
hydraulic conductivity ranged from 970 gpd/ft^ to 2,100 gpd/ft^. The tests 

29 



were not conducted under ideal conditions as Mountain Water Company's pumping 
schedule for MWC34 could not be altered to benefit the tests. 

The distribution of hydraulic conductivity and transmissivity values 
calculated from specific capacity data by Clark (1986), Meyer (1985) and 
Pottinger (1988) are presented in Figures 3.8 and 3.9. One pattern emerges in 
the western portion of the study area where values of transmissivity and 
hydraulic conductivity are lower (Clark, 1986, and Meyer, 1985). However, the 
work (Meyer, 1985) which generated these lower values is poorly documented. 
The distribution of hydraulic conductivity and transmissivity data in the 
Missoula Aquifer also appears to reflect an area of higher hydraulic 
conductivity and transmissivity values near the mouth of Hellgate Canyon and 
adjacent area south and southwest of the river. 

Values for aquifer properties of Unit Three generated for this report are 
similar to values reported by previous workers. Using data from specific 
capacity tests, McMurtrey and others (1965) determined transmissivity values 
which varied from 17,800 to 1,000,000 gpd/ft. From an aquifer test in the 
north central part of the study area, these workers also calculated a 
transmissivity value of 620,000 gpd/ft. An aquifer test by Geldon (1979) 
provided a transmissivity value of 699,927 gpd/ft. Hydrometrics (1984) 
attempted an aquifer test on MWC34, the same well tested by Clark (1986), and 
concluded that transmissivity ranged from 250,000 to 1,000,000 gpd/ft, in good 
agreement with Clark (1986). From 19 aquifer tests, Geldon (1979) reports an 
average hydraulic conductivity value of 5,090 gpd/ft^. In the same test 
series, he determined specific capacity values ranging from 3 to 3,000 gpm/ft. 

Grimestad (1977) reported a hydraulic conductivity range of 3,400 to 
16,660 gpd/ft^ for Unit Three in the central valley area. McMurtrey and 
others (1965) reported that the transmissivity for two wells producing water 
from the same unit ranges from 77,000 to 125,000 gpd/ft. 

McMurtrey and others (1965) assumed a porosity value of 0.40 and a 
specific yield of 0.10. From aquifer tests Geldon (1979) found time-dependent 
specific yield values which ranged from 0.11 to 0.35. 

SUMMARY OF HTDR0L06IC PROPERTIES 

The vertical and horizontal heterogeneities in stratigraphy throughout 
the aquifer contribute to the wide range in hydrologic values. It is 
difficult to generalize about these properties. The most reliable values for 
each property have been selected and are summarized in Table 3.3. The 
rationale for selecting each value is explained below. Generally, it is 
believed that aquifer properties derived from aquifer tests, permeameter 
experiments and driller's log analyses are most accurate. Hydraulic 
conductivity values calculated from sieve analyses were considered too high 
and were rejected because the techniques are intended for sand and not coarse 
sand and gravel. 

The hydraulic properties assigned to Unit One are based on the 
permeameter testing data presented by Clark (1986) and thickness data reported 
by Morgan (1986). Transmissivity values were obtained by multiplication of 

30 




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32 



hydraulic conductivity by thickness. For Unit Two specific capacity and 
saturated thickness data came from Morgan (1986). Hydraulic conductivity 
estimates were obtained by averaging interpretations of Morgan's data for Unit 
Two and Grimestad's (1977) value for his intermediate unit. 

In Unit Three porosity was derived from the permeameter work by Clark 
(1986). Specific yield was estimated from the permeameter data, from aquifer 
tests conducted by Clark (1986), and from McMurtrey and others (1965). 
Aquifer thickness is presented as a range of values based generally on work by 
Clark (1986), Morgan (1986) and Pottinger (1988). Hydraulic conductivity is 
presented as a range. The low value corresponds to permeameter results 
(Clark, 1986) and the higher value represents an average calculated from 
Mountain Water Company well specific capacity data. Vertical hydraulic 
conductivity values come from Clark's (1986) aquifer tests. The selected 
transmissivity values were calculated from specific capacity data. The lower 
value is the average transmissivity computed from all specific capacity data. 
The higher value is the average from specific capacity data from just Mountain 
Water Company wells. This range of transmissivity values is supported by 
Clark's (1986) aquifer tests. 

Single of single values for transmissivity and hydraulic conductivity for 
Unit Three were selected based on data obtained from wells which were drilled 
with cable tool methods, had perforated aquifer intervals with known aquifer 
thickness, and whose pumping tests were at least four hours long. 
Transmissivity values from municipal wells (MWC) best fit these criteria and 
average 1,710,000 gpd/ft. Hydraulic conductivity values average 25,500 
gpd/ft2. 

The values given in Table 3.3 provide a general description of the 
Missoula Aquifer and indicate that the aquifer is highly conductive. Unit Two 
appears to be significantly lower in transmissive ability than Units One or 
Three, although the actual range and spacial variation in Unit Two's hydraulic 
properties throughout the study area are not well understood. Hydraulic 
conductivity and transmissivity values appear extremely high for Unit Three. 
Operation of large production wells in this unit have small drawdowns, which 
support these high values. As an example, well MWC3A pumps at 7,000 gpm and 
typically has less than eight feet of drawdown at the well. MWC23 and MWC35 
are located away from the river and yield 250 and 2,500 gpm with one and six 
feet of drawdown, respectively (Plate 1). 

In an attempt to characterize the aquifer with just a few numbers, the 
hydrologic properties were calculated for a hypothetical composite aquifer 
composed of 10 ft of Unit One, 20 ft of Unit Two and 60 ft of Unit Three. The 
weighted average value for hydraulic conductivity is 18,200 gpd/ft^ and 
1,152,000 gpd/ft for transmissivity. Single values for porosity and specific 
yield are 0.20 and 0.12, respectively. 



CHAPTER 4 
WATER TABLE FLUCTUATIONS 



This chapter addresses three objectives: 1) describe the network of 
observation wells used to measure changes in water table elevation and aquifer 
storage during the study; 2) present and discuss the observed data; and 3) 
evaluate these water level changes in a historical context. 

HONITORIRG WELL HETHORR 

The well monitoring network developed during this study is composed of 
several different groups of wells. In the summer of 1985 the first 31 
domestic wells were chosen and given the MV label. Criteria for selection of 
these wells included access, potential for long term monitoring, availability 
of a driller's log, and location in the flow system. Twenty-nine of these 
wells were monitored monthly until summer 1987, Two wells were monitored 
monthly until summer 1986. Well locations are presented on Plate 1 and in 
Figure 4.1. 

Additional water level data for the valley were obtained from a 48 well 
network (labeled P) used by Pottinger (1988) during a detailed investigation 
in the vicinity of Interstate 90 and Grant Creek (Plate 1) and 24 Mountain 
Water Company (MWC) wells (Figure 4.1). Pottinger (1988) measured water 
levels monthly from summer 1985 to summer 1986. Water level records for most 
MWC wells begin in the mid 1970' s. Typically, measurements were recorded one 
to three times each year until 1984 when monthly water level records were 
kept. .Monthly measurements were made at all MWC wells throughout the duration 
of this project. 

The only well for which there is a water level record longer than the MWC 
wells is MV31 (also MWC7) located in the parking lot of Montana Power 
Company's Missoula office. This well was monitored monthly by the USGS from 
September 1958 until December 1966; quarterly by the USGS until July 1981, 
sporadically by the MWC and USGS from 1981 to 1984, and monthly by this 
project starting in summer 1985. An electrical transducer system capable of 
recording daily water level fluctuations was installed in July, 1986. 

By December 1985, seven additional monitoring wells were constructed as 
part of this project (MV34 to MV40) and were monitored monthly (Figure 4.2). 
Two additional wells (MV41 and MV42) were constructed in summer 1986 as part 
of the storm drain study (Wogsland, 1988). These nine wells were drilled with 
a direct air rotary drilling rig equipped with a drill-through casing driver. 
Ten inch surface casing was set to 20 ft below land surface and six inch steel 
casing was advanced to approximately 10 ft below the water table. Drill 
cuttings were collected at five foot intervals. The wells were perforated 
with a down-hole perforator over a 10 to 20 ft interval approximately centered 
at the water table. Five rounds of perforations were cut. Each perforation 
was 0.10 ft long and 0.01 ft wide. The wells were grouted with a cement 
slurry which was poured in from the top of the ten inch surface casing as this 
ten inch casing was removed. To complete construction, a steel instrument 

34 




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36 



shelter was welded to each well. The two wells completed in 1986 were 
finished with flush coupled PVC casing and 10 ft of 60 slot PVC well screen. 
The six inch steel casing was removed and the drill hole allowed to collapse 
around the screen. Two to three feet of bentonite pellets Were installed 
above the screened interval. Twenty feet of six inch diameter steel casing 
was left in the top of the bore hole to protect the PVC casing and for 
attaching an instrument housing for a Stevens Type F continuous recorder. 
Well logs for each of the nine wells are presented in Appendix 4A. 

All monthly water level data collected as part of this study were 
measured using steel tape. Water levels collected by Mountain Water Company 
were measured using either a tape or air line. Well inventory information for 
wells analyzed as part of this study is found in Appendix 3A. All water level 
data are tabulated in Appendix 4B. 

Casing elevations in feet above mean sea level were surveyed by the 
Missoula County Engineers Office. Elevations are accurate to 0.01 ft. 

One objective of this project was to construct a long term water level 
monitoring network for the Missoula Valley. Well MV31 and seven of the 
project constructed monitoring wells were equipped with electrical transducer 
systems in early summer 1986 (Figure A. 3). The use of transducers for long 
terra monitoring of water level fluctuations Is new. Systems installed in the 
Missoula Valley were modified from designs developed by the Northern Rocky 
Research group of the U. S. Forest Service (Prellwitz and Babbit, 1984). 

RESULTS OF HATER LEVEL MONITORING 

Data from the following wells illustrate the water level trends observed 
during the 1985 to 1987 study period: 

1. MV34, MV35 and MV6 , near the Clark Fork River; 

2. MV31 and MV26, in the central portion of the area east of the Clark 

Fork River; 

3. MVIO, west of the Clark Fork River; 

4. MV39 and MV20, near the Clark Fork and Bitterroot Rivers in the 

western portion of the study area; 

5. MV2 and MV36, at the mouth of Rattlesnake Creek; and 

6. P31, at the mouth of Grant Creek. 

Figures 4.4 to 4.10 show hydrographs for each of these wells. Monthly water 
level data for all wells included in the monitoring well network are presented 
in Appendix 4B. 

In addition to measurements of monthly water levels, daily water levels 
were measured at eight wells equipped with electrical transducers. The 
results of the transducer operation and actual monthly water levels are 
presented in Figures 4.11 to 4.14. Though battery problems affected data 
collection, transducer results are encouraging. The daily records generated 
from the transducer equipped wells support all trends observed from the 
monthly hydrograph data. Records for MV31 and MV36 show that transducer 
systems can be used to accurately record water level fluctuations (Figures 

37 




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39 



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Oet-BS 



Uay-B7 



Figure 4.5: Hydrograph of data from well MV6. 



40 



X 

t 

c 
o 

9 


a 



3.128 - 



MV31 — Montana Power 

Wm Hydro^ropn 




Jiin-B7 



X 

c 
o 
■a 





5.130 



3.129 - 



3.128 - 



5.127 - 



3.126 - 



3.125 - 



Q 3.124- 



3.123 - 



3.122 - 



3.121 - 



MV26 — South Avenue 

Wwi Hy UT OQf opn 




Jun— 83 



D«e-B3 



Jun— 88 



D«e-Be 



I I I 

Jun— 87 



Figure 4.6: Hydrographs of data from wells MV31 (top) and MV26 (bottom) 



41 



MV10 



3.145 

3.144 

3.143 - 

3.142 - 

3.141 - 

i'» 3.140 H 

o « 3.139 H 


|£ 3.138- 
3,137 - 
3.136 - 
3.135 - 
3.134 



3.133 



Feb-B5 




Mor-B6 
Dot* 



Oet-se 



Figure A. 7: Hydrograph of data from well MVIO 



42 



Buckhouse - MV39 



3.123 



3.123 - 
3.122 
3.122 
^^ 3.121 
I S 3.121 H 

U J 

yc- ii2o - 

3.120 - 
3.119 - 
3.119 - 



o 
o 
o 

r- 
X 

t 

c 
o 

□ 



3.11B 



D«e-89 



3.1102 



3.1100 - 
3l1098 - 
i1096 - 
3l1094 - 
5.1092 - 
3.1090 - 
3.1 0B8 - 
3.1086 - 
3.1084 - 
3.1082 - 
3.1080 - 
3.1078 - 
3.1076 - 
3.1074 - 
3.1072 - 
3.1070 - 
3.1068 -I 




Jaf>-B7 



Au«-B7 



Det« 



MV20, 4412 Spurgin Rd. 

W«ll Hydrogroph 



Jun— BS 




Jun— B7 



Figure 4.8: Hydrographs of data from wells MV39 (top) and MV20 (bottom). 



43 



o 
o 
o 



Q 



3.232 



3.231 - 
3.230 - 
3.229 - 
3.228 - 

3.227 - 

3.228 - 
3.225 - 
3.224 - 
3.223 - 
3.222 - 
3.221 - 
3.220 - 
3.219 - 
3.218 - 



3.217 - 



MV2 - 1250 Monroe 

W«ll Hydrofraph 



?>-^ 




Jun-89 



Dee— 85 



Jun-86 



Dm-BS 



Jiin-B7 



Dec~85 



Greggory Park - MV36 



3.198 - 




' \ 






3.197 - 










3.197 - 








/\ 


^^ 3.198 - 

Is 3,196- 

1 ** 

QC' 3.195 - 






1 


J 


3.195- 






■\ / 




3.194 - 






\/ 




3.194 - 
3.193 - 




1 


\/ 


1 



Jul-88 



Jon— 87 



Au«-B7 



Dot* 



Figure 4.9 Hydrographs of data from wells MV2 (top) and MV36 (bottom) 

44 



Well #P31 



c 
o 

a 

2 



3.1 3» - 
1158 - 


A 








3.157 - 


/ \ 






;' 


3.158 - 


/ \ 








3.155 - 


/ \ 








3.154 - 


/ \ 








3.153- 


/ \ 








3.152 - 


/ \ 








3.151 - 


/ A 








3.150 - 


1 \ 






■ 


3.149 - 


\ 








3.148 - 


\ 








3.147 - 


H 






y 


3.146- 


\ 






/ 


3.145 - 


\ 




\ 


Ji 


3.144 - 


\ 




\ 


/ 


3.143 - 


\ 




\ 


/ 


3.142 - 






^•--^ 


/ 


3.141 "■ 


• 1 


1 


1 ' 


' 1 



U4iy-B9 



Ju>-B5 



S«p-BS 



^lo^^-85 



Jon-B6 



Uw^BS 



Figure 4.10 Hydrograph of data from well P31 



45 



S.162 



MADISON ST. BRIDGE - MV34 

Well Hydro gfo ph 




May-B6 



Tnnnduew 





D«o-B6 



Jijn-B7 



o 

o 
o 



X 

c 
o 
9 
o 

□ 



5.16 



3.1 58 - 
5,156 - 
3.154 - 
3.152 - 

5.15- 
3.148 - 
3.146- 
3.144 - 
3.142 - 

3.14 - 
3.138 - 
5.136 - 



5.134 



Mar-B6 



MV35. McCormick Park 

Well Hydregroph 




% 




1 

jui-Be 



— I — 

Oct-B6 



1 

Jon— B7 



May-B7 



Au«-B7 



IVonsduoer * Steel tope 

Figure 4.11: Hydrographs of transducer data from wells MV34 (top) and MV35 
(bottom) . 



46 



8.142 
3.141 - 
5.140 - 
8.139 - 
8.138 - 

8.157 - 

5.158 - 
8.138 - 
8.134 - 
8.138 
8.132 H 



8.131 



MV-37, Reserve Street 

Well HydroBfoph 





-86 



— I r 1 

Jul-B8 0«f-88 



T 1 1 1 

Jon— 87 May-B? 



Aus-B7 



Trontif u««r 



« SImI Top* 



o 

o 
o 

r* 
X 

t. 
C 

o 

3 
O 

□ 



5.14 



5.128 



Jur>— 86 



MV31, Montana Power 

W«JI Hydrogroph 




•^ 



Aug— 86 



Ort-Se 



Dm-88 



F«b-B7 



Figure 4.12: Hydrographs of transducer data from wells MV37 (top) and MV31 
(bottom) . 



47 



3.12B 



3.127 - 



^\ 


5.120 


r' 




K '■■ 




s 


3.125 


C 

o 

I 




3.124 



3.123 



3.122 - 



Jul-86 



MV40. State Lands 

W«n Hydrov«ph 




T 1 

Ort-B8 



1 

Jan-B7 

IMTE 



lyonnducar 



llay-S7 
Sl««i Top* 




T r 



AU9-B7 



3.122 



3.116 



Jul-B6 



MV39. Buckhouse Bridge 

Wdl Hydregraph 




Ocl-B6 



Au«-B7 



DATE 



Figure 4.13: Hydrographs of transducer data from wells MV40 (top) and MV39 
(bottom). 



48 



3.106 



1105 - 



o 
o 



t, 3.103 - 

c 
o 



i 



□ 3.102 - 



3.101 - 



MV38, SOUTH AVENUE 

W«il Hydrograph 




T 

Oct-86 




Jan— 87 



T 1 

May-B7 



Au«-87 



o 
o 
o 



X 



3.200 -r 



3.199 - 



3.198 - 



3.197 - 



I 3.196 - 
3.195 - 

3.194 - 



3.193 



M«-86 



MV36, Greggary Park 

Well Hydroyoph 




TRANSDUCER 



DATE 



^ Sl««l top* 



Figure 4.14: Hydrographs of transducer data from wells MV38 (top) and MV36 
(bottom) . 



49 



4,12 and 4.14), A number of the sites sustained battery failure In the 
winter. At other sites, minor operational problems were not recognized 
because of the field staff's lack of experience. This resulted in partial 
loss of daily records at the remaining stations. Transducers at MV40, MV34 , 
MV35 and MV37 appeared to work well before power loss. Stations MV38 and MV39 
required modification to obtain acceptable records. 

Five significant trends are shown in Figures 4,4 to 4,14. The first is 
an annual water level rise which occurs between March and June and then a 
general decline until the following February or March, Table 4,1 summarizes 
the measured water level rises from the winter low to the spring peak. 
Transducer data were used in conjunction with monthly measurements to derive 
1986 changes at MV34 and MV36 , The spring rises were 6,5 to 12 ft in 1986 and 
7.5 to 10 ft in 1987 at wells located immediately adjacent to the Clark ForK 
River (MV34, MV35 and MV6). MV31 is located further away from the Clark Fork 
River and showed spring water level rises of seven feet in 1986 and eight feet 
in 1987. MV26 increased about six feet during the spring of both years. The 
water table north of the Clark Fork River rose about eight feet at MVIO during 
both 1986 and 1987. Water levels at MV6 and MVIO were also collected prior to 
spring peak in 1985. These data show a decline of about eight feet from the 
peak to March. Spring water table rises at MV20 were about two feet each year 
and at MV39, two and one half to three feet. At the mouth of Rattlesnake 
Creek spring rises were at least four feet in 1986 and two feet in 1987 at 
MV36. Well MV2, located about a quarter of a mile up the creek, showed a 
three feet rise in 1986 and a 13 ft rise in 1987, This well has a peak water 
table position in the spring, and the lows occurred in late winter in 1986 and 
late fall in 1987, It is unclear what caused the drop and recovery of water 
levels in the fall of 1986, Possibly this water level reflects pumping of 
adjacent wells. Though only partial data are available to define the 1985 
peak, the water table at P31, located at the mouth of Grant Creek, declined 
approximately 17 ft from the high measured in 1985, 



TABLE 4,1 
Net Water Table Rises from Winter to Spring Peak 

(feet) 

Well 1986 1987 

MV34 12* 10 

MV35 
MV6 
MV31 
MV26 
MVIO 
MV20 
MV39 
MV2 
MV36 
(*Transducer data used to define peak) 



6,5 


9 


8 


7.5 


7 


8.5 


6 


6 


8.5 


8 


2 


2 


2.5 


3 


3 


13 


4 


2 



50 



The second trend is defined by a decrease in the elevation of the spring 
peak water level at all wells in 1986 than in 1987. Minimum water table 
elevations were also lower in 1987 than 1986. Water table peaks at MV6 and 
MVIO were less than one foot higher in 1985 than 1986. The third trend is 
observed in seasonal water table fluctuations; the fluctuations decrease with 
increasing southward from the Clark Fork River, Fourth, wells located in the 
Missoula Aquifer near the mouths of Grant Creek and possibly Rattlesnake Creek 
show the largest annual fluctuations in water table elevation (Figures 4,9 and 
4.10). Finally, water levels in wells located near the Bitterroot River are 
correlated with flow in the Bitterroot River and indicate that the ground 
water and river systems are linked. Maximum water level rises are on the 
order of three feet for wells located adjacent to river (Figure 4,8). 

These trends probably reflect seasonal variation in recharge to the 
aquifer and increased withdrawal for public water supply. The maximum water 
table values observed between May and July correlate with the Clark Fork River 
stage as shown in Figures 4.15 and 4.16. Losses measured by Clark (1986) in 
stream discharge between the University Walking Bridge and the Reserve Street 
Bridge indicate that the river recharges the aquifer. Other evidence that 
recharge occurs from the Clark Fork River includes potentiometric maps which 
show that ground water flows to the north and south away from the river 
(Chapter 5) , river stage elevations which are higher than the adjacent water 
table (Figures 4.15 and 4.16), and water level fluctuations at wells close to 
the river channel which are greater than those at wells farther from the river 
(Figure 4.17). 

The water table declines naturally in response to seasonal reductions in 
Clark Fork River discharge and corresponding decreases in ground water 
recharge and to continued natural discharge from the system to the Bitterroot 
River and lower reaches of the Clark Fork River (Figure 4.18). The elevation 
of the water table at well MV39 is higher than the stage of the Bitterroot 
River, which indicates flow is maintained to the river from the aquifer. In 
addition to natural discharge from the system, ground water is withdrawn from 
individual, industrial and municipal wells. Although withdrawals vary 
seasonally, most total annual withdrawals have remained constant since the 
late 1970's. The one exception is the withdrawal of water from the aquifer by 
Mountain Water Company. Figure 4.19 presents the total monthly ground water 
withdrawal by the Mountain Water Company well network (Appendix 4C). Until 
1983, just over 4 billion gallons of water were used each year, with the 
largest production occurring in the summer. After the abandonment of the 
Rattlesnake surface water supply in summer 1983, annual ground water 
withdrawal has been over 9.7 billion gallons. Peak monthly demands since 1983 
have exceeded 1.2 billion gallons. 

HISTORICAL WATER TABLE TREHDS 

The water level data collected and compiled as part of this study 
represent the most extensive documentation of water table trends in the valley 
to date. However, in order to evaluate the long term trends in water table 
elevations, it is necessary to assess records collected over as many years as 
possible. The records for well MV31 (MWC7) provides water level data from 
1958 to the present. This well is not pumped. 

51 



S.157 



3.156 - 
3.155 
3.154- 
3.153- 
i 3.152 H 

s? 

I i 3.15 H 
^ 3.149 H 
3.148- 
3.147 - 
3.146- 
3.145 - 
3.144 



Madison St. Well - MV34 

Stotfe Wotor Uv«| v*. 1bn« 






7/83 



11/88 



3/86 



7/86 



11/86 



3/87 



7/87 



3.1776 



Clark Fork River ~ North Walking Bridge 




11/8S 



-I — »" 
3/86 




3/87 



7/87 



Figure 4.15: 



Hydrographs of data from well MV34 (top) and the Clark Fork 
River at the University Walking Bridge (bottom). 



52 



t C 



3.142 
5.141 
3.14 
3.139 
3.1 38 
3.137 
3.136 
3.13S 
3.134 
3.133 
3.132 
3.131 



RESERVE ST. WELL - MV37 

STATIC WWtR LEVEL VS. TIME 



/ 





I t t I ■ 1 1— —J r- 



T p— r- 



I • 1 I I • »~ 



7/83 11/83 3/88 7/88 

UONTTORINC MOHmS 



11/86 



3/87 



CLARK FORK RIVER - RESERVE ST. N.BRIDGE 



STAGE vs. TIME 



3.1455 



3.145 - 



i1445 - 



3.144 - 



1^^ 



3.142 



3.1415 





—I r— I 1 1 r- 

7/B5 1 1/85 



T 



» < 1 1 I I 



-I 1 t 



3/86 7/86 

UOWrORINC MONTHS 



11/86 



3/87 



Figure A. 16: Hydrographs of data from well MV37 (top) and the Clark Fork 
River at the Reserve Street Bridge (bottom). 



53 



CLARK FORK RIVER - NORTH WALK BRIDGE 

STAGE vs. TIME 




«5 5.1 782 
*^| 5.1760 
!i g i175B 
^£5.1756 



7/B5 11/B5 3/B6 7/B8 

UONITORINO MONTHS 



11/B8 



3/87 



7/B7 






9.16 



S.15- 



5.14- 



3.13- 



5.12 



3,11 - 



Well Hydrographs 



3.1 -f 




7/B5 



11/85 



3/88 



7/88 



11/88 



3/87 



7/87 



amv34 



rnv6 



mv13 



mv16 



mv18 



mv20 



Figure 4.17: Hydrographs of data from the Clark Fork River (top) and wells 
located progressively further from the river (bottom). 



54 



BUCKHOUSE BRIDGE WELL - MV39 

STATIC WATKR LEVEL VS TME 



i123 






5.1225- 




5.122- 




5.1215- 


t c 

■ " 


5.121 - 
5.1205 - 


5.12- 




11195- 




5.119 - 




5.1 1B5 - 



7/BS 11/85 3/B6 7/88 

uoNrroRiNC months 



11/86 



3/87 



BITTERROOT RIVER - BUCKHOUSE BRIDGE 



STAGE vs. TWE 



5.1195 




5.118 



5.1155 



-I • I 1 r— — t 1 I I r— I 1 1 I 1 I I 

11/B5 3/88 7/88 11/88 

UONfTCRINC MONTHS 
Figure 4.18: Hydrographs of data from well MV39 (top) and the Bitterroot 
River at Backhouse Bridge (bottom). 



55 




i I I I I 

n n ^t n N •- 



> 



q(i|i)N«<)'tnN.-oz 
^6666666666 



c 

CO 

a 

§ 
u 

u 

0) 

u 

(0 



cd 

c 

O 

CO 

r-i 

i 

u 



u 

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c 
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o 
u 
bO 



4:: 

e 
o 

e 

td 
u 
o 

H 



I— I 
•H 



(suoflog JO suojigg) pujpiflijii 



56 



The water level trends observed in Figure 4.20 show a general fluctuation 
between about 3,146 ft to 3,132 ft from 1958 through 1980. From 1980 to 1985 
water level data were collected at times which would not reflect periods of 
highest and lowest water levels so trends are difficult to distinguish with 
these data. Since 1984, monthly water level data show maximum and minimum 
values fluctuating between about 3,140 and 3,130 ft. 

Review of historical water level records from MWC wells shows trends 
similar to those of MV31. Figures 4.21 and 4.22 present typical hydrographs 
for Mountain Water Company wells. The water level trends for wells MWC34 and 
MWC30, which are located adjacent to the Clark Fork River, fluctuate about ten 
feet seasonally. They both show an apparent stepwise lowering in water levels 
in late 1983. A similar, but less pronounced, trend is seen in wells MWC19, 
located north of the river, and MWC20, located to the south. All of these 
wells are active pumping wells in the MWC network. 

Analysis of the long term water level trends in the valley provides an 
indication of how the aquifer is responding to climatic influences and pumping 
stress. The hydrographs presented to illustrate the longer terra trends show 
what appears to be a general lowering of the valley water table since 1983. 
This trend can be seen by looking at the lower elevations of yearly water 
table raaxirauras and minimums between 1983 and 1987. This record is too short 
to adequately assess whether a long term trend of net annual decline is 
actually occurring. This apparent lowering of the water table can be 
attributed to the Increased withdrawal which Mountain Water Company started in 
1983 (Figure 4.19) and below average Clark Fork River annual and peak 
discharges which also began in 1983 and continued through 1987 (Figure 4.23). 
It is not possible with just water level trend data to separate entirely the 
individual affect of each of these factors. The source or sources of water 
level decline will be examined using a mass balance approach in Chapter 5. 



57 



0+ & 



> 




to 



n 



to 



tOK)tOfOtO»0»OIO»OIOtOtO 



o 

2 



U -J 



o 

3 



O 



fo 



a 
o 
u 

CO 

td 
-a 



a 
to 

bO 

o 

33 



O 
CM 



<1> 

u 

•H 
[14 



(spuosnoLii) 



58 



MWC34 



il 

a ■ 

>? 
o r 

E »- 



3.162 
3.161 

3.16 - 
3.159 - 
3.158 - 
3.157 
3.156 - 
3.155 - 
3.15* - 
3.153 - 
3.152 - 
3.151 - 

3.15 - 
3.M9 
3.1*8 - 
3.147 - 
3.M6 - 
3.1*5 
3.1** - 
3.1*3 
Jan 





-78 



1 1 1 1 1 1 

Oct-80 Jul-83 Mar-86 D«o-8B 

Dot* 



MWC30 



Z o 




DATE 



Figure 4.21 Hydrographs of data from wells MWC3A (top) and MWC30 
(bottom). 



59 



MWC19 




3.151 



3.1 38 



Apr-75 



MWC20 




— I — 

Jon— 78 



1 

Oct- 80 



"1 1 

Jul-83 



1 r 

Mar-86 



D«e-B8 



DATE 



Figure 4.22: 



Hydrographs of data from wells MWC19 (top) and MWC20 
(bottom). 



60 




• 

9 



L 

2 
*> 
c 



2 




{■p)a6jD3S)a 



ntnNr-toiaNVAtnNfnaian 
•tii-t't'-nnnrinrtnnn.-WNo 
•: r *:':*: ri 1 ''.':';':•■.'■.•;•: n •: *: 

(spuonRnu) 

(VJUODBMO 



CO 

CM 



u 

3 
bO 



61 



CHAPTERS 
GRODHD WATER FLOW: SOURCES OF RECHARGE AND DISCHARGE 



Chapter 4 described the monitoring well network established during this 
project and the variations in water table elevations which occurred in the 
Missoula Aquifer. This chapter describes aquifer recharge and discharge, 
general ground water flow paths and seasonal variations in the flow field. 

GROUND WATER FLOW SYSTEM 

Monthly water table elevations collected at over 50 wells were plotted 
and contoured to produce maps showing the configuration of the top of the 
saturated zone in the aquifer. Three of these potentiometric maps are 
presented (Figures 5.1 to 5.3) as examples of water level fluctuations and 
variations in flow paths during 1986. The months selected were: 

1. March, when the water table was at its lowest for the year; 

2. June, when water levels were highest; and 

3. October, as water levels were declining. 

The same general pattern of ground water flow is maintained during most 
of the year. North of the Clark Fork River, ground water moves away from the 
channel and away from the northern aquifer boundary, where ground water from 
the Tertiary sediments and the alluvium in the Grant Creek and Rattlesnake 
Creek Valleys recharges the aquifer. The net result is ground water migration 
parallel to the river channel and final discharge to the Clark Fork River 
north of the river's confluence with the Bitterroot River. South of the river 
ground water flows southwest towards the Bitterroot River and towards the 
confluence of the Clark Fork and Bitterroot Rivers. 

By comparing the water table elevations adjacent to the Clark Fork River 
with a river channel profile derived from USGS topographic maps. It is evident 
that the stream is influent over a reach downstream of Hellgate Canyon (Figure 
5.4). The length of the influent reach varies seasonally, from about four 
miles during June and July when the water table is at its highest and over six 
miles during the winter and spring. Figure 5.4 also shows that the largest 
differences between the elevations of the river channel and water table occur 
in the first three to four miles below Hellgate Canyon. Water level trends 
discussed in Chapter 4 and water quality data presented in Chapter 6 also 
indicate that the Clark Fork River recharges the aquifer. 

Variations in the rate at which the Clark Fork River and other sources 
recharge the aquifer are reflected in the buildup of the water table in the 
late spring and early summer, and its subsequent decline from mid summer 
through late winter. A decrease in recharge from late June to the following 
March caused the 3,135 ft equipotential line to migrate over two miles to the 
northeast (Figure 5.5). The almost identical position of the this 
equipotential line southwest of the first two miles of river channel from 
December through March probably implies that aquifer recharge and discharge 
relationships have stabilized. During the same period, additional migration 

62 




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of the 3,135 ft equipotential line Is observed north of the river in the 
vicinity of the Reserve Street Bridge. This trend is most likely controlled 
by a decline in recharge from the influent Grant Creek and a continuing 
seasonal reduction in recharge from the Tertiary sediments to the north. 

AQUIFER RECHAR6B AND DISCHARGE 

Water recharges the Missoula Aquifer from a number of sources: 

1. Influent reaches of the Clark Fork River; 

2. Direct precipitation on the aquifer; 

3. Discharge from the adjacent Tertiary Sediment and Bedrock 

Hydrostratigraphic Units; 

4. Storm water runoff; 

5. Septic systems; and 

6. Leakage from irrigation ditches. 

Leakage from the Clark Fork River in the valley is the major source of 
recharge to the Missoula Aquifer. Clark (1986) attempted to quantify the 
recharge from a three mile reach of the river by measuring the loss in 
streamflow between the University Walking Bridge (about 0.5 mi from the mouth 
of the Hellgate Canyon) and the Reserve Street Bridge, After accounting for 
irrigation diversions in this reach and inflow from Rattlesnake Creek, he 
concluded that about 14% of the flow measured at the Walking Bridge, or 
324,700 acft/yr, leaked through the streambed and recharged the aquifer 
(Figure 5.6). Clark's (1986) calculations were further refined by 
incorporating the variations in the length of the influent reach of the river 
and by estimating monthly instead of annual seepage losses (Figure 5.7), The 
revised estimate of leakage from the river is 412,700 acft/yr. Leakage 
calculations are explained in Appendix 5A. 

The role of direct precipitation on the unconfined aquifer has not been 
quantified. If precipitation contributes to aquifer recharge at all, it 
probably occurs only in the spring associated with snow melt and spring 
rainfall. By July, all rainwater not entering the Missoula storm water 
systems is probably evapotransplred. From November to March, the ground is 
frozen and recharge from the surface does not occur. 

Spring precipitation and snowmelt compose the principal recharge to 
bedrock and Tertiary sediments which underlie surrounding highlands. Water 
from these hydrostratigraphic units then recharges the Missoula Aquifer 
through lateral inflow. Equipotential lines indicate a source of recharge 
from the mountainous terrain north of the valley (Figures 5.1 to 5.3). These 
sediments do yield small volumes of water to wells; equipotential surfaces in 
these sediments are higher than in the Missoula Aquifer. 

The annual ground water discharge from the hills on the aquifer's 
northern boundary was estimated by assuming an aquifer thickness of 75 ft, a 
hydraulic conductivity of 100 ft/d and a hydraulic gradient of 0.006 for this 
6.75 mi boundary (Pottinger, 1988). Calculated discharge is 13,400 acft/yr. 



68 



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Some minor recharge may also come from the hills on the east and southeast of 
the valley and from underlying formations. These sources were not quantified. 

Ground water flow from other small stream valleys is an additional source 
of recharge in the northern portion of the aquifer. Sendler (1986) estimated 
that 350 acft/yr of ground water discharges from the Rattlesnake Creek 
alluvium to the Missoula Aquifer, du Breuil (1983) estimated ground water 
recharge from the Grant Creek Valley at 4,900 acft/yr. Smaller streams, such 
as Pattee Creek and Butler Creek, also are influent as they enter the Missoula 
Valley. Their recharge rates were not quantified. 

Storm water runoff in the Missoula area is channeled into over 2,000 dry 
wells which allow water to percolate to the water table. Wogsland (1988) 
estimated that 365 acft of water are injected annually. 

Septic systems service all but the main Missoula metropolitan area. Over 
5,700 individual residences are found in the valley with an additional 600 in 
the Rattlesnake Creek Valley. Ver Hey (1987) measured daily loading of about 
250 gpd per household. Based on her research, about 1,600 acft/yr of septic 
wastes recharge the aquifer. 

Geldon (1979) attempted to quantify the seepage from irrigation ditches 
and irrigated areas for the immediate area around Missoula. Though values 
have not been derived for the entire aquifer, his results provide an 
approximation good to an order of magnitude. He estimated that irrigation 
practices account for approximately 8,500 acft/yr of aquifer recharge. 

In the Missoula area, losses from Mountain Water Company's distribution 
system are estimated to be over 12,250 acft/yr, assuming a 50 percent loss of 
the water produced. Table 5.1 summarizes the sources of recharge to the 

Missoula Aquifer. 

I 

Water which discharges from the aquifer by evapotranspiration and as base 
flow to streams has not been quantified. Evapotranspiration is probably an 
important source of aquifer discharge in riparian areas along streams and from 
subirrigated fields west of the confluence of the Bitterroot and Clark Fork 
Rivers . 

Ground water is pumped from wells owned by individuals, industrial users, 
and two private water companies, Mountain Water Company and Clark Fork Water 
Company (CFWC) . For the last two years MWC and CFWC have produced an average 
of over nine billion gallons per year from the aquifer. About 45,000 people 
live within MWC's service area and on the average use 550 gallons per person 
per day (MCCHD, 1987). This daily use rate is high because of the 50 percent 
water loss from the distribution system. The Clark Fork Water Company serves 
2,329 people, who each use about 360 gallons daily. An estimated 4,700 
families are served by single or multi-family wells outsideof the water 
company service areas. Each family is assumed to use about 320 gpd (MCCHD, 
1987). Total annual withdrawal from wells is estimated at 30,210 acft. 



71 



1,600 


0.4 


350 


0.07 


365 


0.08 


4,900 


1.0 


8,500 


1.9 


12,300 


2.7 


13,400 


3.0 


412,700 


90.9 



TABLE 5.1 
Volume Estimates of Recharge to the Missoula Aquifer 

Source Amount 

acft/yr % of total 

Septic systems 
Rattlesnake Creek Valley 
Storm Water* 
Grant Creek** 
Irrigation*** 
MWC line loss 
Lateral inflow, north 
Clark Fork River 

TOTAL 454,115 

(*Missoula area) 
(**No other creeks quantified) 
(***0nly in the immediate Missoula area) 



The volume of natural discharge from the Missoula Aquifer was 
approximated (assuming no net change in storage) by subtracting the estimated 
annual recharge to the aquifer (Table 5.1) from the annual withdrawal from 
wells (Table 5.2). The estimated annual discharge is 423,905 acft. 



TABLE 5.2 
Water Use from the Missoula Aquifer 

Population Use Per Capita 

Daily (gpd) Annual (acft/yr) 

Mountain Water Company 44,755 550 27,570 

Clark Fork Water Company 2,329 360 940 

Households with 4,700 320* 1,700 



private wells 

(*gallons per household) 



Total 30,210 



In summary, aquifer recharge exceeds estimated ground water discharge and 
withdrawal by 15 times. The leakage of water through the bed of the Clark 
Fork River accounts for over 90% of the annual recharge. Therefore water 
table declines noted in Chapter 4 most likely reflect recent climatic 
conditions which have caused reductions in recharge rates from the river and 
from Tertiary sediments. Water levels may decline locally in areas of heavy 
pumping, particularly during periods of low recharge, but in general, current 
levels of ground water withdrawal do not appear to be over drawing the 
aquifer. 

72 



CHAPTER 6 
AQUIFER HATER QUALITY 



This chapter includes a discussion of sampling methodology, general 
aquifer chemistry, results of an intensive sampling of the aquifer west of 
Reserve Street and summaries of three ground water quality research efforts 
conducted in conjunction with and partially support by this study. 

METH(H>S 

Twenty-six monitoring wells, six Mountain Water Company wells, and two 
locations on both the Clark Fork and Bitterroot Rivers were sampled five times 
on a quarterly basis between February 1986 and May 1987 (Figure 6.1). All 
samples were analyzed for gross ionic chemistry by Dr. Juday (Department of 
Chemistry, University of Montana). Energy Labs of Billings, Montana, 
determined dissolved trace metal content of samples from lA of the 26 sites. 
Standard sampling and analytical procedures were exercised. Analytical data 
are presented in Appendix 6A and 6B. 

The quality control program utilized duplicates, blanks, and standards. 
Three sample duplicates and three blanks were added to each group of 26 
samples delivered to the laboratories for gross ionic analysis. Each batch of 
trace metal samples had one blank, one duplicate, and one trace metal standard 
(prepared from National Bureau of Standard's SRM i!fl643b.) 

The last quarterly sampling (spring 1987) included an expanded set of 
monitoring wells. Sixty-eight domestic wells located west of Reserve Street 
plus 28 sites in the quarterly monitoring network (Plate 1) were sampled for 
gross ionic chemistry and coliform bacteria (Figure 6.2). Gross chemistry 
analyses were performed by Dr. Juday. Coliform analyses were completed by the 
Missoula City County Health Department. Sixty-six dissolved trace metal 
analyses were done by Dr. Moore (Department of Geology, University of 
Montana). Sampling followed standard procedures. A quality control program 
similar to the one described above was used. 

In June 1987, six ground water samples were collected and analyzed for an 
EPA priority pollutant scan, which includes analyses for pesticides, PCB's, 
and purgable, acid extractable and base neutral organic compounds. Standard 
procedures were followed when sampling and analyses were performed using EPA 
standard procedures by Lancaster Laboratories of Lancaster, Pennsylvania, 
Blanks and duplicates were not included for quality control because of the 
high cost of analysis and the limited sampling budget. 

RESULTS: GEHERAL DESCRIPTION OF GROUND WATER QUALITY 

The ground water in the Missoula Aquifer is a calcium bicarbonate type. 
Near the Clark Fork River, the mineral content of ground water fluctuates 
seasonally in response to changes in river water quality. Throughout the rest 
of the aquifer, mineral content remains essentially constant at any one point 
but increases in the direction of ground water flow. Detectable metal 

73 




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concentrations occur primarily in Clark Fork River water and in adjacent 
wells. Stiff diagrams indicate the relative proportion of major ions in 
ground water throughout the aquifer (Figure 6.3). Average and representative 
water analyses are shown in Tables 6.1 and 6.2. 



TABLE 6.1 

Water Analyses of Representative Missoula Valley 

Ground Water Samples Collected February 5, 1986 

(mg/1) 



Parameter 


Up gradient 


Down gradient 




MV34 


MV25 


Bicarbonate 


177.9 


199.3 


Chloride 


2.46 


5.55 


Sulfate 


19.3 


22.1 


N03 (as N) 


0.41 


1.13 


Calcium 


41.3 


47.0 


Magnesium 


11.6 


14.4 


Sodium 


5.9 


7.4 


Potassium 


1.3 


2.0 


TDS 


261.6 


302.6 



TABLE 6.2 
Average Water Quality for All Water Analyses 
(mg/1, n = 222) 



Parameter 


Average 


Minimum 


Maximum 


Bicarbonate 


187.4 


77.5 


237.8 


Chloride 


3.85 


0.52 


13.2 


Sulfate 


21.5 


1.4 


40.5 


N03 (as N) 


0.76 


0.1 


1.9 


Calcium 


44.9 


16.5 


57.1 


Magnesium 


12.9 


1.9 


18.5 


Sodium 


6.97 


2.7 


12.1 


Potassium 


1.78 


0.7 


2.6 


TDS 


282.7 


113.4 


357.8 



Std. Deviation 

26.5 

1.6 

6.2 

0.4 

5.8 

2.0 

1.2 

0.3 
35.9 



Seasonal variations in water quality occur in the Clark Fork River and 
nearby wells. The fluctuations water chemistry in these wells and the river 
(Figure 6.4) are synchronous both temporally and in magnitude and, therefore, 
provide confirmation that the aquifer is recharged by the river. At distances 
over one half mile from the river, ground water quality remains fairly 
constant with time at any one well (note wells MV16, MV27 and MV20 in Figure 
6.4). 



Metals are primarily found in the Clark Fork River and in wells near the 
river. Copper is the exception and was detected randomly in about one third 

76 




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MISSOUU VALLEY AQUIFER 



TOTAL DISSOLVED SOLDS 



2fO 



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MISSOULA VALLEY AQUIFER 



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Hydrographs showing seasonal variation in TDS for the Clark 

Fork River and adjacent wells (top) and seasonal change In 

TDS along a ground water flow path from CFNWB to MV20 

(bottom). 

78 



of the wells throughout the valley. With one exception (MV38) , measured raetal 
concentrations did not exceed drinking water standards. The Clark Fork River 
had detectable levels of arsenic, zinc, and copper in November 1986, but at 
other times all metals were at or below detection levels. Wells MV3, MV8 and 
MVIO had elevated levels of zinc and copper. Well MV38 had a manganese 
concentration of 0.17 mg/1 and was the only well sampled with manganese above 
the detection level. These four wells are all within one quarter mile of the 
Clark Fork River. Their elevated levels of metals may be related to 
unidentified metal contaminated sediments in the Clark Fork River floodplain. 
However, there are many other wells in close proximity to the river which do 
not have detectable levels of metals. Concentrations of lead, mercury, and 
cadmium are all at or below their detection levels. 

Results of trace metal analyses should be viewed with caution and only as 
trends because all sampled wells (except MV41 and MV42) had steel casings and 
most samples were collected through the household plumbing. To reduce the 
potential of raetal contamination, samples were collected only after the well 
and piping system had been flushed with fresh ground water and the water 
temperature and specific conductance had stabilized. 



CHEMICAL TRENDS: RESULTS OF SPRING 1987 SAMPLING 

Early in the project a review of aquifer stratigraphy and water quality 
data indicated that the western portion of the aquifer may be susceptible to 
contamination from individual sewage disposal systems. This area of the 
aquifer was not well represented in the water quality sampling network and 
therefore, an expanded quarterly sampling was carried out in May and June of 
1987. Sixty-six additional domestic wells which were volunteered by their 
owners for sampling were added to this round of sample collection (Figure 
6.2). The resulting chemical data are extensive (Appendix 6B) and were used 
to construct isoconcentrational maps of the aquifer and examine trends in 
water quality. 

Water quality steadily decreases in the down gradient direction from 
Hellgate Canyon east and southeast toward the Bitterroot River. For example, 
total dissolved solids increase from a low of about 240 mg/1 to a high of 358 
mg/1 over distances of four to five miles. The distribution of total 
dissolved solids in the Missoula Valley measured during spring 1987 (Figure 
6.5) is very similar to that of the other four sampling periods. Comparison 
of a potentioraetric map (Figure 5.2) and the isoconcentrational map (Figure 
6.5) shows that the steady increase in total dissolved solids occurs in the 
direction of ground water flow. The down gradient increase in total dissolved 
solids is also apparent in Figure 6.4. 

The observed down gradient increase in total dissolved solids is most 
likely caused by natural dissolution in the aquifer of carbonate minerals, 
such as calcite and dolomite. The increase in calcium, magnesium and 
bicarbonate concentrations correlates with the increase in total dissolved 
solids. Concentrations of other major ions, such as sulfate, sodium, and 
chloride, do not correlate with total dissolved solids. 

79 




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Chloride concentrations in ground water are of interest because of the 
potential impact on water quality from road salting and septic tank drain 
fields. Average chloride concentration is 3.85 mg/1 (Table 6.2) and, as found 
for total dissolved solids, the general trend is an increase in concentration 
in the down gradient direction (Figure 6.6). Elevated concentrations occur Ln 
several places. Two zones of higher chloride concentration appear north of 
the Clark Fork River. One area is near the Russell Street Bridge and the 
other is associated with well MWC30 (Figure 4.1). Both are probably 
restricted to the north side of the river by the hydrologic divide created by 
recharge from the river. The source of higher chlorides north of the river is 
not known at this time. South of the river elevated concentrations of 
chloride are found in near the Champion Mill and California Street. Septic 
systems are the probable cause of part of the elevated chlorides found in this 
area. A third area with elevated chlorides is located in the southern most 
portion of the area between the Bitterroot River and well MWC14. In all 
sampling periods well MWC14 had chloride concentrations of 10 to 12 mg/1, 
levels which are about three times the average (Table 6.2). The higher 
chloride could be originating form sewage disposal by household septic systems 
or from natural recharge from the Tertiary sediments to the south. The final 
area of elevated chloride Is in the western part of the valley, where some 
concentrations about 25% above background were found. The sources of excess 
chloride are most likely septic systems in areas which are not served by the 
city sewer system (Ver Hey, 1987). 

Nitrate concentrations exceeding 2.0 mg/1 were not detected in the wells 
sampled as part of this study. Generally, nitrate concentrations increase in 
the down gradient direction (Figure 6.7). Areas of apparent elevated nitrates 
correspond with areas of elevated chloride. Based on the available data set, 
the western portion of the aquifer has large areas with nitrate concentrations 
between 0.8 and 2 mg/1. There are no known natural sources of nitrate in the 
aquifer which can account for the levels found. It was originally thought 
that storm water injection by over 2,000 storm drains in the urban area may be 
loading nitrate into the system. However, Wogsland (1988) concluded that 
storm water injection did not appear to be adding to nitrate to the system. 
Ver Hey (1987) concluded, from a study of the affects of septic systems on the 
ground water in the western portion of the aquifer, that ground water 
contamination from both chloride and nitrate was occurring. Though this study 
did not detect concentration over about 2.0 mg/1, other researchers have found 
high concentrations of nitrate associated with some individual wells in the 
western portion of the aquifer and in subdivisions in the southwestern portion 
of the valley (Howard Newman, personal comm., 1988). 

Sixty-six wells were sampled for dissolved trace metal concentrations. 
All concentrations were below EPA drinking water standards (Appendix 5B). 
Most concentrations were below instrument detection limits. 

Water samples collected during the spring 1987 sampling period were 
analyzed for coliform bacteria (Figure 6.8). Eighteen out of the 98 water 
samples tested positive for bacteria. All but one of the positive samples 
came from wells west of Reserve Street. The one positive sample east of 
Reserve Street came from observation well MV34, which was never disinfected 
after completion and has not been used for water supply. 

81 




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EPA priority pollutant scans were completed on samples from six wells 
(Appendix 6C) . In the 112 analyses performed on each sample, all constituents 
were below the instrument detection limit with the few exceptions listed in 
Table 6.3. All detectable concentrations are lower than recommended drinking 
water standards. The PVC casing in wells MV41 and MV42 probably caused the 
contamination found in these wells. The origin of the methylene chloride (a 
common laboratory solvent) in two of the samples is unknown. 



TABLE 6.3 
Results of EPA Priority Pollutant Scans 

Well Compound above detection limit Concentration 

(ug/1) 

MV34 none 

MV42 bis (2-ethylhexyl) phthalate 40 

#41 bis (2-ethylhexyl) phthalate 80 

//28 none 

#33 methylene chloride 22 

#60 methylene chloride 20 



EXAMPLES OF GROUND WATER CONTAMINATION 

This section illustrates the vulnerability of the Missoula Aquifer to 
contamination by spills, leaks and area wide percolation of water from a 
number of small sources. Brief summaries of research on a herbicide release, 
the storm drainage system and septic tanks are presented. 

A. Herbicide Release 

In the fall of 1984, the Montana Department of Agriculture discovered 
trace amounts of piclorara (0.052 ug/1) and 2,4-D (0.9 ug/1) in a well serving 
the Missoula County Weed Control Facility (MCWCF) (Figure 6.9). They also 
found picloram (2.4 to 4.5 ug/1) in two wells serving a commercial trailer 
court and campground. The most likely source of these herbicides was the 
MCWCF waste sump, which was found to be contaminated with picloram, bromacil, 
2,4-D and 2,4,5-TP (Figure 6.10). Herbicides were also found in the soils 
around and beneath the sump drain. The Montana Water Quality Bureau 
determined that the herbicide levels found in area wells were orders of 
magnitude lower than available drinking water standards but still recommended 
additional analysis of the site. 

Pottlnger (1988) assessed the ground water flow system and the transport 
of herbicides from the MCWCF during 1985 and 1986. He found that picloram and 
bromacil were the only herbicides found in measurable concentrations in an 
area covering over one and one half square miles (Figure 6.11). The 
distribution of the contaminants was controlled by water table variations of 
over 20 ft and by a seasonal shift in the ground water flow direction from 
south in the spring and early summer to southwest by west in midwinter. These 
changes in the ground water system are caused by seasonal variations in 

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Figure 6.11 



General area in which measurable concentrations of picloram 
and bromacil were detected in the ground water (Pottinger, 
1988). 



88 



discharge to the Missoula Aquifer from the Grant Creek Valley and from Grant 
Creek as it flows into the valley over the Missoula Aquifer. Pottinger (1988) 
successfully constructed a numerical ground water flow model accounting for 
seasonal variation in recharge and developed a solute transport model to 
predict the migration of the herbicides. Figures 6.12 and 6.13 show the 
modeled position of the herbicide contaminated ground water in 1974 and 1989. 

In summary, Pottinger (1988) demonstrated that contaminating the aquifer 
at one small point (less that 100 ft^ in size) resulted in the spreading of 
the contaminant over 1.5 rai^ . His work also showed that small portions of the 
aquifer could be successfully modeled. 

B. Storm Drain Recharge 

Missoula has no valley wide storm sewer system. Instead, runoff from 
streets percolates directly into the aquifer from over 2,000 street corner 
drains. Wogsland (1988) measured the quality and quantity of urban runoff in 
residential and commercial areas in the valley and then, using two sites 
instrumented with soil water and ground water sampling devices, examined the 
impact of storm water disposal on aquifer water quality (Figure 6.14). 

Wogsland (1988) found that, with the exception of runoff from spring 
snowmelt, street runoff entering the aquifer is low in total dissolved solids 
and free of EPA priority pollutants. Commercial areas, however, do generate 
runoff with water quality that is worse than that from residential areas. As 
the water migrates through the vadose zone and towards the aquifer, it reacts 
with earth materials and increases in mineral content (Figure 6.15). Runoff 
from snowmelt runoff contains higher total dissolved solids and sodium 
chloride than runoff from other sources. These higher concentrations appear 
to diminish as the water passes downward through the vadose zone. Figure 6.16 
shows the impact of the contaminated snowmelt runoff on ground water quality 
directly beneath instrumented drains in a residential and a commercial area. 
The chloride concentration at the commercial site exceeds background by over 
four times during recharge by snowmelt. The data also show that the 
concentration of iron in the ground water is usually below detection except 
when chloride rich snowmelt waters reach the water table (Figure 6.17). This 
implies chlorides may be facilitating the transport of metals in runoff and In 
the vadose zone to the aquifer. Figure (6.18) shows the variation in nitrate 
concentration in the ground water at the commercial site. The variations in 
nitrate levels do not appear to be correlated with storm water injection but 
appear to vary in response to changes in the regional flow system. 

Wogsland (1988) also sampled existing wells In the commercial area 
between South Avenue and US 93. Though her work is not conclusive, it appears 
that total dissolved solids and metal concentrations are higher than 
background levels in this part of the aquifer. 

Wogsland's (1988) research documents the importance of snowmelt runoff 
and dissolution of vadose zone materials in determining the water quality in 
the aquifer. She concluded that the vadose zone supplies major ions to the 
aquifer and attenuates metal migration except during spring snowmelt recharge. 
Further installation of sump storm drains will contribute to minor aquifer 

89 




0.0021 mg/1 



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Figure 6.12: Modeled distribution of herbicides In 1974 (Pottlnger, 1988). 



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Figure 6.13: Modeled distribution of herbicides in 1989 (Pottinger, 1988) 



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at a commercial site and a residential (Wogsland, 1988). 



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contamination. But, more importantly, Missoula's storm drains provide an open 
conduit to the aquifer for any contaminants or hazardous wastes spilled on 
city streets. 

C. Domestic Sewage Recharge i 

Two household septic systems located in the western part of the valley 
(Figure 6.19) were instrumented and studied in 1985 and 1986 by Ver Hey 
(1987). An access port was installed in each septic tank and soil water 
samplers were installed at two depths in the vadose zone below the drain - 
field. Water meters were used to determine the rate at which waste entered 
the septic system. Wells were used to sample the quality of the underlying 
ground water (Figure 6.20). Both sites were located in coarse soils 
classified as Grantsdale loam. All legal requirements for septic system 
installation had been met at each site. The water table varied seasonally 
between 8.5 and 14 ft below land surface. 

The study showed that the septic drain fields are relatively ineffective 
in treating household effluent due to the coarse grained soils in which the 
systems were installed (Table 6.4). Approximately 200 gallons per day per 
household enter each system, but the water retention time in the coarse soils 
is so short that all effluent percolates out of the drain field within 15 ft 
of the septic tank. Also no biological mat was found during excavation of the 
drain fields. Water analyses show that no statistically significant reduction 
in phosphorus or nitrogen concentrations occurs as the effluent percolates 
from the drain field to the water table. A plume of nitrate and chloride 
could be traced down gradient from the drain field at both sites (Figure 6.21 
and 6.22). Fecal coliform numbers were above the maximum detectable limit in 
ground water samples taken directly below the drain field and were still high 
(eight bacteria per 100 ml) 50 ft down gradient (Figure 6,23). The presence 



TABLE 6.4 

Septic System Treatment as Mean Per Cent Removal of Effluent 

Constituents, Year Two (Ver Hey, 1987) 



Specific 
Conductance 



Total Ortho- 
Nitrogen Phosphate 



Sodium Chloride 



Site H 



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35 
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of a 0.2 inch amphipod in a ground water sample suggests that void spaces are 
large in the underlying aquifer. Samples of ground water taken up gradient of 
the study site have elevated concentrations of chemical constituents and 
possibly indicate wide spread degradation of ground water quality by 
individual septic systems in the area. 

Ver Hey (1987) concluded that traditional septic system design is 
ineffective in treating domestic waste in the coarse alluvial soils of the 
Missoula Valley. She cautioned that the continued use of existing systems and 
the addition of new systems would result in degradation of existing ground 
water supplies. 



103 



CHAPTER 7 
GROUHD WATER FLOH MODELING 

INTRODUCTION 

Construction and calibration of a transient model of ground water flow 
through the Missoula Aquifer proved to be a difficult task. Although 
significant progress was made in defining the distribution of aquifer 
properties and boundaries, and in understanding the interaction between the 
Clark Fork River and the ground water system, we were unsuccessful in 
developing a complete numerical model which could be used for predictive 
purposes. Future modeling efforts based on this work and coupled with 
collection of additional data will be able to describe the responses of the 
aquifer to variations in Clark Fork River recharge and to pumping by Mountain 
Water Company. The following discussion of ground water modeling is taken 
from unpublished work by Brick (1987). 

MODEL DESIGN 

A two-dimensional model was developed for ground water flow in the 
aquifer for one year starting in October 1985 and ending in September 1986. 
The area covered is shown in Figure 7.1. The finite difference computer code 
named PLASM developed by Prickett and Lonnquist (1971) and subsequently 
adapted by Prickett for the IBM PC series computer was used. Several 
modifications were made to this program to tailor it to this study. Data for 
the model were taken from past and ongoing work, especially that of Clark 
(1986) and Pottinger (1988). 

The Missoula Valley and the grid for this model are bounded by Mt. 
Sentinel to the east. Waterworks Hill to the north, the South Hills and 
Bitterroot River to the south and thick Lake Missoula sediments of low 
permeability to the west. The grid consists of 38 columns and 31 rows with 
variable node spacing. The density of nodes is greater over the population 
center of Missoula and the locations of municipal wells (Figures 7.1 and 7.2). 
The boundary conditions for the grid are illustrated in Figure 7.3. The east 
and west boundaries are no-flow boundaries. A constant head is assigned to a 
portion of the north boundary because flow maps and previous work by Geldon 
(1979) indicate a small source of water from the north, possibly from the 
Tertiary sediments. Pottinger (1988) also used a constant head boundary in 
this area. The model was modified to allow the constant head at the north 
boundary to change seasonally because a seasonal change in head is indicated 
by potentiometric maps of the aquifer (Figures 5.1 to 5.3). The constant head 
at this boundary was set to vary between 3,142 ft and 3,155 ft. The south 
boundary is defined by the Bitterroot River and is assigned a constant head 
determined by the river's stage and gradient. This boundary is discussed 
further below. 

Apart from the model's boundaries, initial input parameters include the 
storage coefficient, hydraulic conductivity and the thickness of the aquifer. 
In each of the two modeled dimensions, the aquifer is divided into blocks 

104 




Figure 7.1: Grid for finite difference model (after Brick, 1987) 



105 




Figure 7.2: Model grid and location of Mountain Water Company wells 
(after Brick, 1987). 



106 



Grant Creek 



North boundanrconstant head 



Rattlesnake Creek 




constant head 



Figure 7.3: Model boundary conditions and locations of wells used to 
evaluate calibration (after Brick, 1987). 



107 



which are bounded by two adjacent nodes and which have constant hydraulic 
properties - Heterogeneity can be modeled by assigning different hydraulic 
conductivity and storage values to each of these blocks. Hydraulic 
conductivity and storage coefficient cannot vary with depth because the model 
Is two-dimensional. The storage coefficient in this model is 0.10, which is 
close to the 0.12 value measured in permeameter tests and to storage 
coefficient values (0.03 to 0.10) determined from several aquifer tests 
(Clark, 1986), 

The hydraulic conductivity distribution used in the model was based on 
previous research and Figure 3.8. The model itself was also used to evaluate 
hydraulic conductivity values. The final distribution of values is 
illustrated in Figure 7.4. It should be emphasized that this distribution of 
hydraulic conductivity values was chosen because the results of the model fit 
known steady state water level data and not because it necessarily represents 
reality. 

The bottom elevation of the modeled aquifer is represented by the contour 
lines in Figure 7.5. This configuration is based on the work of Morgan (1986) 
and Figure 3.2. 

Additional input parameters include the location and temporal variations 
of major water sources and sinks. These include the withdrawal from municipal 
wells and infiltration from streams (Clark Fork River, Rattlesnake Creek and 
Grant Creek) . The selection of Clark Fork River recharge rates is discussed 
below. Rattlesnake Creek and Grant Creek are treated as Injection wells where 
they enter the valley. The amount of water injected was determined from the 
work of Sendler (1986) and du Breuil (1983). Rattlesnake Creek is represented 
by one injection well while Grant Creek is represented by eight wells which 
extend into the valley. The monthly injection schedules for these wells are 
in Table 7.1. These sources provide a minimal amount of water except during 
runoff in May and June. 

TABLE 7 . 1 

Schedule of Injection Wells Used to Simulate 

Rattlesnake Creek and Grant Creek 

(gpd per node) 

Month Rattlesnake Creek Grant Creek 

Oct 10,000 50,000 

Nov 10,000 50,000 

: . Dec 10,000 50,000 

Jan 10,000 50,000 

Feb 50,000 100,000 

Mar 100,000 400,000 

Apr 200,000 600,000 

May 300,000 900,000 

Jun 400,000 900,000 

Jul 100,000 200,000 

Aug 10,000 100,000 

Sep 10,000 200,000 

108 




Figure 7.4: Distribution of hydraulic conductivity used in the model 
(after Brick, 1987). 



109 



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Figure 7.5: Elevations of the base of the aquifer used in the model 
(after Brick, 1987). 



110 



Pumping schedules for municipal wells were obtained from Mountain Water 
Company. The monthly production rates were converted to a daily rate and then 
divided in half to account for the assumed 50 percent loss by leakage from 
distribution lines (Table 7.2). This leakage recharges the aquifer, and, for 
all practical purposes, reduces pumpage by half. MWC well locations are shown 
on the modeling grid in Figure 7.2. 

TRANSIENT FLOW SIMULATION 

Downward leakage of Clark Fork River water is the main source of recharge 
to the Missoula Aquifer and therefore the primary driving force of the ground 
water flow system. The recharge rate is dependent on the stage of the river 
and the leakage rate through the river bed. The flow system responds less 
strongly to the stage of the Bitterroot River, discharge from Grant, 
Rattlesnake Creeks and pumping of large yield wells and variation in lateral 
flow from Tertiary and Frecambrian bedrock which bounds the aquifer. 

In order to model the aquifer over a one year period, it was necessary to 
periodically change the stage of the Clark Fork and Bitterroot Rivers and to 
change the leakage rate of the Clark Fork River. To accomplish these 
conditions, the PLASM code was modified to read new values of head (stage) and 
leakance each month. Average monthly discharges of the Clark Fork River were 
calculated from the USGS gaging data recorded at the station above Missoula 
(Figure 2.3). Monthly discharges of the Bitterroot River were estimated by 
subtracting Clark Fork River discharge measurements at the gaging stations 
above and below Missoula. These values compared favorably to several 
discharge measurements made on the Bitterroot River by Clark (1986). 
Discharges were converted to river depth or stage (Figure 7.6) using stage- 
discharge relationships developed for each river by Clark (1986). River stage 
elevations were then correlated with river profiles determined from USGS 
topographic maps. 

The leakage of the Clark Fork River bed is known to vary, possibly in 
response to seasonal changes in the bed's permeability (Clark, 1986). Leakage 
was calculated from Clark's measurements of discharge lost from the river. 
Though estimated in Chapter 5, the average rate of leakage is not known for 
each month. Based on available data, leakage rates from to 5 gpd/ft-^ at 
each river node of the model were selected to initiate modeling. Since the 
value of leakage was not well determined, a number of different values were 
tested in the model. The final values used are in Table 7.3. 

The transient model simulation begins in October 1985 after the model has 
reached steady state conditions. The steady state heads are used as initial 
heads for October. The model then reads new head and leakance values for the 
Clark Fork River, new head values for the Bitterroot River, new injection 
rates for wells simulating Grant Creek and Rattlesnake Creek, new discharge 
rates for the pumping wells, and adjustments for recharge from the northern 
model boundary. A new set of simulated heads is then calculated for the end 
of the month. This process continues for 11 months until completion of the 
simulation at the end of September 1986. 

HI 



CLARK FORK RIVER AVERAGE MONTHLY STAGE 



AT WALKINO BR1D0E, CFNm 



S.I 21 




S.114 



118ft- 1181 



BITTERROOT RIVER AVERAGE MONTHLY STAGE 



AT BVCXHOUSE BRIDGE. BBBR 



B.178 
8.1778 - 
8.1778 - 
8.1774 - 
8.1772 - 

8.177 - 
8.1788 - 

t'J 3.1788 - 
g I 8.1784- 
5 I 3.1782 - 
£ 8.178 - 
8.1788 - 
8.1788 
8.1784 H 
8.1782 

8.178 - 
8.1748 - 
8.1748 



§ 




Figure 7.6 



Hydrographs showing average monthly stage used in the model 
for the Clark Fork River (top) and the Bitterroot River 
(bottom) (Brick, 1987). 



112 



TABLE 7.2 

1985-1986 Pumping Schedules for 

Mountain Water Company Municipal Wells 

(gpd) 



MWCl 



MWC2 MWC3,37 



MWC7 



MWC8 



MWC9 



MWCIO 



Oct 


56307 


37 


398742 


2278655 


65 





62339 


Nov 


34049 


89 


604226 


636263 


49 





59597 


Dec 








348403 


581033 


162 








Jan 








412968 


641031 


129 





182629 


Feb 








275565 


609965 











Mar 








293903 


677771 


65 








Apr 








746774 


655636 


65 


162 


44291 


May 


618194 


50918 


1262532 


659812 


25097 


3113 


653807 


Jun 


653791 


200184 


279307 


671618 


88194 


13355 


639984 


Jul 


503758 


161202 


1192516 


677912 


92548 


2500 


634226 


Aug 


530081 


198329 


1115790 


621865 


103887 


13613 


647484 


Sep 


187145 


3608 





655852 


81 


97 


630710 



MWCll 



MWCl 4 



MWCl 6 



MWC18 



MWCl 9 



MWC20 



MWC21 



Oct 


97 


607403 


49 


81 


307 


145 


113 


Nov 


25920 


574452 


81 


97 


129 


145 


65 


Dec 


19049 


81 


129 


145 


613 


613 


226 


Jan 


680420 


586855 





145 


129 


62371 


35694 


Feb 


903210 


530984 








49 


56533 


35694 


Mar 


1004435 


557129 


65 


162 


194 


37065 


145 


Apr 


968452 


567710 


81 


65 


371 


160087 


97 


May 


994742 


556726 


42178 


49 


40129 


364645 


304258 


Jun 


684984 


575242 


142952 


5678 


84774 


433516 


325645 


Jul 


323887 


589178 


104307 


3016 


99420 


426178 


455339 


Aug 


986016 


594694 


182984 


9258 


190920 


516710 


904597 


Sep 


346307 


511484 


49 


65 


65 


61403 


55403 



MWC22 MWC23,24 



MWC25 



MWC26 



MWC27 



MWC29 



MWC30 



Oct 


97 


265008 


10420 


49 


65 


98839 


1791 


Nov 


97 


251123 


14742 


65 


65 


90920 


113 


Dec 


65 


266299 


13710 


97 


162 


95791 


1016 


Jan 


65 


231207 


97 


210 


49 


110662 


2304920 


Feb 


1065 


219426 





65 





100420 


2062322 


Mar 


6033 


250621 


920 


65 


65 


119887 


2307322 


Apr 


11710 


567710 


8420' 


81 


65 


134242 


2200161 


May 


16516 


326033 


82565 


523581 


11484 


192081 


2275193 


Jun 


62194 


347307 


247065 


478629 


50291 


273145 


1566242 


Jul 


49484 


343581 


239033 


370371 


20452 


279339 


728452 


Aug 


60533 


369820 


322420 


731984 


43920 


335355 


996274 


Sep 


162 


282431 


13500 


43629 


49 


117662 


59807 



113 



MWC31 



TABLE 7.2, continued 
MWC32 MWC33 MWC3A 



MWC35 



MWC36 



Oct 


2404516 


1628129 


226 


3530806 


194 


3113 


Nov 


2323500 


1155371 


274 


3525967 


178 


65 


Dec 


2406435 


1070435 


323 


3826613 


274 


97 


Jan 


14871 


1555742 


237758 


3031935 


161145 


355 


Feb 


4242 


1478952 


4242 


3061774 


113 


468 


Mar 


10645 


1692774 


291 


3550806 


113 


952 


Apr 


564678 


1265435 


145210 


3023226 


6371 


12936 


May 


586420 


1783887 


752855 


1165000 


161420 


77645 


Jun 


1765032 


2039742 


1074580 


3475645 


65 


234936 


Jul 


2413258 


2031677 


1050338 


4040484 


178 


303750 


Aug 


2387984 


2078258 


1231355 


3735161 


162 


644581 


Sep 


2356532 


1794613 


68936 


3464193 


162 


18081 



TABLE 7.3 

Leakage Values Used in the Model 

(gpd/ft3) 



Oct 


.8 


Apr 


2.0 


Nov 


.8 


May 


5.0 


Dec 


.3 


Jun 


5.0 


Jan 


.3 


Jul 


1.0 


Feb 


.8 


Aug 


.8 


Mar 


1.0 


Sep 


.8 



MODEL RESULTS 



Despite numerous attempts and manipulations to the model, the simulation 
is still not calibrated to the actual head data. It provides a good, general 
overview of ground water flow in the valley but is not sufficient for 
predictive modeling. 

Examples of the distribution of heads generated by the model are 
illustrated in Figures 7.7 to 7.9. Figures 7.10 to 7.11 are hydrographs which 
illustrate the modeled change in head over time compared to actual measured 
values at selected locations on the grid. 



The transient simulation is not a 
water table. The model more accurately 
of the aquifer than in others. For ins 
7.11) shows the correct fluctuation pat 
simulated heads is one to two feet high 
other problem areas are shown in Figure 
show simulated heads fluctuating either 
to measured heads. Another problem in 



perfect reproduction of the actual 

simulates actual heads in some regions 
tance, the hydrograph for MV22 (Figure 
tern but the overall elevation of 
er than actual heads. Examples of 
s 7.10 and 7.11. These hydrographs 

too much or not enough in comparison 
the model is the head distribution near 



114 



OCTOBER 




WATER TABLE CONTOURS, FTf 3000 
MODELED 
MEASURED 



Figure 7.7: Modeled and measured water table elevations, October 1985 
(after Brick, 1987) . 



115 



MARCH 




WATER TABLE CONTOURS, FTf 3000 
MODELED 
MEASURED 



Figure 7.8: Modeled and measured water table elevations, March 1986 
(after Brick, 1987). 



116 



JUNE 




WATER TABLE CONTOURS, FTf 3000 
MODELED 
MEASURED 



Figure 7.9: Modeled and measured water table eleva 
Brick, 1987). 



tions, June 1986 (after 



117 



WELL MV-3 



1S5 



^•% 

o 
o 
o 
n 

I 

K^ 

z 
o 

I 




modalad raaults 



-4- fl«ld m«amir«m«nfa 



WELL MV-31 



o 

5 



140 



139 - 



13B 



^ 137 - 



o 
o 
o 
n 



4, 136 - 



135 



134 



133 



132 -1 




mod«l«d rcaulta 



flald mMiMiranMnti 



Figure 7.10: Hydrographs showing modeled and measured heads at wells MV3 
(top) and MV31 (bottom) (Brick, 1987). 



118 



WELL MV-22 



O 

o 
o 
n 

I 

«^ 

z 
o 



116 



115 




110 -f 



D medalad raiuKa 



•*■ fl«ld m«aturwn«nla 



Figure 7.11: Hydrographs showing modeled and measured heads at well MV22 
(Brick, 1987). 



119 



Grant Creek. Although the model injects more water than is really warranted 
by the data, the heads in this area are consistently too low. 

The spacial distribution in modeled heads matches measured data best in 
March (Figure 7.8). Modeled water tables for October and June are poor 
representations of the actual head distribution (Figures 7.7 and 7.9) One of 
the problems encountered was that the model fails to simulate the wide 
seasonal and spacial changes in the water table illustrated in Figure 5.5. 
This figure shows the migration of the 3,135 ft water table contour from June 
to March, a trend not reproduced by the model. 

The model is imperfectly calibrated largely because the necessary input 
parameters are imperfectly known on a valley-wide scale. The Missoula Aquifer 
is a large heterogeneous aquifer with many sources and sinks of water. 
Although many parameters have been quantified for the Missoula Aquifer, more 
continuous data are needed to effectively use the model. To date, missing 
data have been filled by approximations. In particular, a better picture of 
the distribution of hydraulic conductivity is needed. Figure 3.8 shows a wide 
range of conductivity in the aquifer, however, the actual distribution of the 
various fluvial sediments comprising the aquifer is poorly known. Morgan's 
(1986) effort to delineate the distribution of hydraulic conductivity was the 
most detailed but the problem needs to be considered in more detail. 
Evaluation of the aquifer using a geologic facies model is needed. However, 
extensive drilling and sampling of the aquifer would be required to build a 
detailed model. 

Until more data are obtained from the aquifer, the best approach toward 
modeling may be to work at a different scale. Smaller segments of the aquifer 
have been modeled successfully (Pottinger, 1988; Peery, 1989) as the problems 
of large scale heterogeneity are less acute. Smaller models may ultimately be 
more useful for analyzing specific problems and making specific conclusions. 
As more information becomes available it should be possible to eventually 
model the entire valley. 



120 



CHAPTER 8 
CONCLUSIONS AND RECOMMENDATIONS 

The results of this two year study lead to the following conclusions: 

1. The Missoula aquifer is stratigraphically complex. It is composed 
of three units, the first or second of which is not always present. The 
upper most unit, Unit One, is 10 to 40 ft thick and is composed of 
interbedded boulders, cobbles and gravel. The middle zone. Unit Two, is 
composed of up to 40 ft of tan to yellow silt with sand and gravel. 
Unit Three, the basal unit, is composed of 50 to 100 ft of interbedded 
gravel, sand and silt. 

2. The hydrologic properties of the aquifer reflect site specific 
stratigraphy and depositional environments. Values of hydraulic 
conductivity and transmissivity appear to decrease south and southwest 
of the Hellgate Canyon and Grant Creek area. The hydraulic conductivity 
of Unit One and Unit Three ranges from 10,300 gpd/ft^ to 25,000 gpd/ft^. 
Values for Unit Two average 200 gpd/f t^ . Transmissivity values for Unit 
One and Unit Three range from 103,000 gpd/ft to 1,710,000 gpd/ft. 
Values for Unit Two average 8,000 gpd/ft. Approximate hydraulic 
properties of the entire aquifer, assuming the aquifer is acting as one 
homogeneous unit, are a porosity of 0.20, specific yield of 0.12, 
hydraulic conductivity of 18,200 gpd/ft^ and a transmissivity of 
1,152,000 gpd/ft. -: 

3. Changes in aquifer storage are indicated by water table variations. 
These fluctuations reflect seasonal changes in the quantity of recharge 
reaching the aquifer and in the rates of ground water withdrawal by 
pumping and by natural discharge. During 1985-1986 and 1986-1987 wells 
closest to the influent portions of the Clark Fork River, Rattlesnake 
Creek and Grant Creek showed water table fluctuations of three to 17 
feet. Wells located further from these points of recharge fluctuated at 
a lower amplitude. During 1985-1986, peak water table elevations 
throughout the valley were higher than peak elevations in 1986-1987. 
The water table low recorded in late winter was lower in 1987 than the 
previous year. These valley wide trends appear to have begun in 1983, 
when climatic conditions apparently changed, resulting in less than 
normal Clark Fork River spring discharge and aquifer recharge. Also 
since 1983, the main water producer in the valley. Mountain Water 
Company, more than doubled its pumping rate in order to increase its 
production of ground water to supply 100 percent of its needs. A step 
function decline in water level occurred at those Mountain Water Company 
wells which experienced the increased pumping. 

4. The Clark Fork River is a losing stream and recharges the aquifer 
from four to six miles of its channel during much of the year. Mass 
balance calculations show that the river accounts for 90 percent of 
aquifer recharge. Total aquifer recharge is 15 times greater than 

121 



withdrawal from Mountain Water Company wells, Clark Fork Water Company 
wells and approximately 4,700 individual wells. Based on these data, 
the apparent valley wide decline in the water table since 1983 is a 
result of a reduction in recharge caused principally by lower than 
normal flow in the Clark Fork River. 

5. North of the Clark Fork River, the direction of ground water flow is 
strongly influenced by recharge from the boundary foothills on the north 
and from the Clark Fork River. Flow in this part of the aquifer 
parallels the river and moves west until it passes the Reserve Street 
area, where it turns south to discharge to the Clark Fork River below 
its confluence with the Bitterroot River. In the area of Missoula south 
of the Clark Fork River, ground water flows south and southwest to 
discharge to the Bitterroot River and, seasonally, to the lower reaches 
of the Clark Fork River. 

6. The water quality of the aquifer is good and does not require 
treatment prior to use in most areas. The water is dominated by calcium 
and bicarbonate ions, is low in total dissolved solids (less than 500 
mg/1) and is similar in chemistry to the Clark Fork River. Wells 
located adjacent to influent streams reflect the chemistry of these 
sources of recharge. As the ground water flows away from these recharge 
areas, total dissolved solids increase through pollution and aquifer 
dissolution. The presence of chloride and nitrate concentrations which 
are elevated above natural background levels suggests degradation from 
anthropogenic sources is occurring. Injection of storm water was found 
to increase chloride concentrations at the water table beneath storm 
drains. The disposal of sewage by seepage rings or drain fields appears 
to be degrading water quality in portions of the valley as both nitrate 
and chloride concentrations are elevated in areas served by septic 
systems. Fortunately, concentrations do not exceed drinking water 
standards. However, coliform bacteria and fecal coliform bacteria have 
been found in individual water supplies at levels which exceed drinking 
water criteria. The aquifer is extremely vulnerable to contamination 
because it is unconfined and consists of generally coarse material. 

7. An attempt to numerically simulate the aquifer's complex 
stratigraphy and recharge-discharge relationships was unsuccessful. The 
computer model could not be calibrated using field data and then 
independently verified with a second set of field data. Problems were 
viewed as being partially attributable to inadequate definition of 
recharge rates from the Clark Fork River and of the distribution of 
aquifer properties. 

The conclusions listed above imply that the Missoula Aquifer is an 
aerially extensive, thin, unconfined system which is geologically complex and 
vulnerable to contamination. It is Missoula's sole source of water and an 
irreplaceable resource. Protection of the Missoula Aquifer requires 
continuing long term observation and management. The following 
recommendations are based on the conclusions outlined above: 



122 



1. Maintain a long term water level observation network. Analyses of 
historical water table fluctuation data were critical in deciphering the 
relationships between apparent water level declines and causes for such 
trends. However, previous data collection efforts failed to measure 
true annual minima and maxima of the water table so trends were 
difficult to define. Secondly, years of daily water level records 
extending over a number of years with differing climatic conditions are 
needed to calibrate and verify a numerical model of the system. The 
establishment of a water level monitoring network has been initiated as 
part of this project. Nine wells drilled specifically for water level 
trend observation were installed. Seven of these wells and well MV31 
(MWC7) have been equipped with electrical transducers and electronic 
data storage systems. The existing system should be expanded by 
installing similar water level recording devices at the remaining two 
wells, MV41 and MV42. In addition, water level records for the region 
north of the Clark Fork River should be reviewed, and one to two wells 
in this area should be instrumented with similar water level recording 
systems. MWC wells would be suitable if permission for installation can 
be arranged. Funding is required to maintain the equipment, collect 
monthly manual water level readings and to reduce and report the data. 

2. Along with a water level monitoring network, a water quality 
monitoring network should be established. Approximately 30 wells 
located throughout the aquifer and finished in various aquifer units 
should be sampled four times a year. Water quality analyses should 
include gross ions, metals and metaloids, and coliform bacteria. 
Yearly, selected wells should be sampled for organic analyses, including 
volatiles, pesticides and solvents. The results of this sampling would 
be used to identify trends in water quality and provide a baseline from 
which the effects of point and non-point contaminant sources can be 
evaluated. Funding is required to pay for water quality analyses, 
sample collection and equipment, and data reduction and reporting, 

3. The development of a numerical model of the aquifer should be 
continued. Long term management of the aquifer as the sole source of 
water supply for Missoula and surrounding areas requires the ability to 
test the effects of increased pumping and changes in recharge rates 
prior to their occurrence. A calibrated and verified model will provide 
aquifer managers with such a capability. In addition to water supply 
management, a model could be used to predict pathways and directions of 
contaminant migration and to identify sensitive portions of the aquifer 
which require special management consideration. Developing a 
computerized model will require additional data for calibration 
including water level records collected over the last year and better 
estimates of the distribution of hydraulic conductivity and Clark Fork 
River leakage rates. Aquifer testing of 60 to 100 wells should be 
conducted to provide an independent check on the thousands of pumping 
and drawdown data recorded by drillers. Additional mass balance work to 
estimate Clark Fork River recharge should be attempted. Funding will be 
required to refine and improve modeling efforts, to support additional 
data collection and to maintain and update the model once it is 
operating. 

123 



It is the goal of these recommendation to provide the citizens of Missoula 
with the facts needed to make educated management decisions regarding the 
future of their source of potable water. Halting data collection and further 
work on model development would be short sighted and could lead to poor 
planning and crisis situations in managing this critical resource. 



124 



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125 



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126 



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127 



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